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you have to choose your own parts really, you're the one that knows what you want/need. We can comment on your choices and advise you.
Regardless, some rules for building a gaming PC:
Less is more
16GB ram will give little additional benefit over 8GB, but don't go lower than 4GB. More than quad core will give no benefit for gaming. (note, for high performance multithreaded apps, such as video 3d modelling/animation, you want as much ram as you can throw at it)
more cores doesn't help, faster cores will. Don't get anything less than 3.0Ghz. (note, for high performance multithreaded apps, such as video rendering/3d animation, go for cores over core speed)
Tons of ram doesn't help, fast ram does. DDR3 is a must, get the highest speed supported by your board and processor. Lower timings are better. The two most important are: CAS Latency and recharge time. The lower the better, however higher mhz takes precedence. Pay attention to quad/triple/dual channel support on the board. Triple channel means RAM must be installed in groups of 3 identical modules, dual channel in groups of two, and quad in groups of 4. Mess this up and it runs much slower. The more modules you have, the better (so long as you don't break the dual/triple/quad channel rule). If you have a board with 6 ram slots and you want 6GB of RAM, go for 6x 1GB over 3x 2GB. This is because of the recharge time, which is a period after a module has been written in which it cannot be written to again. More RAM modules means there will be other RAM modules available while others are recharging, making the system overall more responsive. G-Skill Ripjaw RAM is awesome for gaming, but watch out for the tall heatspreaders. Don't forget that you will need a 64bit OS for more than 4GB of RAM. Kingston valueram is usually OK, but performance memory is made by mushkin, geil or gskill
Hard disks are easily the slowest part of a computer (excluding optical and floppy drives) and there isn't a whole lot that can be done about this. SSDs look great on paper but realworld performance is lacking.
You'll want SATAII/SATA3gb/s, unless you're very rich and hardcore, in which case U320 SCSI and SAS are options (bear in mind these two have a maximum drive capacity of 300GB for SCSI, SAS can go higher). You want at least 7,200RPM rotational speed, and avoid 'green' or 'eco' drives as these run at 5,400 or 5,900 RPM and even if they claim equivalent performance to 7,200s, they aren't equivalent. WD Velociraptors are good performance wise but are known to have reliability issues. Also watch out for cache sizes, where you get a choice of cache size for a specific disk capacity, go for the higher one. SATAIII/6gbps doesn't help much with mechanical hard disks.
Seek time should be as low as possible, and the drive should be one of: Seagate, hitachi, western digital or samsung. If you're going for SCSI or SAS then fujitsu becomes an option. For optimum performance, you want at least three disks. One for installing your OS and programs on, one for your pagefile, and one for installing games on. It's quite common to use a western digital raptor/velociraptor for the latter, but you could also use one for the former, or even for all three (if you're using SATA). If you're going to download a lot of stuff then get a fourth hard disk (a high capacity one, such as a samsung spinpoint F3) Note that partitioning does not provide any performance benefit because it's on the same physical disk.
RAID: avoid RAID 0. It brings very little performance benefit and comes at the cost of data security. I personally use RAID 1 on my OS disk (RAID 1 is where two identical disks are mirrored to look like one disk, and data is written to both, so if one dies then the other takes over). Avoid 'onboard' raid on your motherboard, if you want raid then get a proper raid card (adaptec, LSI logic, mylex or 3ware) avoid 'zero channel raid'.
Optical drive: mostly irrelevant as far as performance is concerned.
CPU cooler: This is hugely important. Don't use the cooler that came with your CPU. It will be adequate, but only just. This is especially true of intel coolers. Frostytech do regular reviews of cpu coolers, Wander over here:
http://www.frostytech.com/ to take a look. Also pay attention to the noise level, you don't want it to be too high or it will be irritating. (I'm deaf so I couldn't care less).
Case: This is also hugely important, but not for the reasons most people think of. Get a fancy looking case if you like, but looks are a secondary concern. Most important is airflow/thermal performance. You want 120mm fans in the front and the back at minimum (or slots to put said fans), the more the merrier, and the bigger the better. Side panel fans are a double edged sword, they can improve airflow, or they can totally balls it up. It depends on the case design. Make sure none of the fanciness blocks fans. Front fans should draw air into the case, back fans should pull it out. Side again depends on case design. For the record I hate tooless cases, its faster to use a damn screwdriver. If the case has a full height drive rack, then you will need a full tower (not a mid tower) otherwise the graphics card wont fit in the case.
Fans: the higher the CFM for a given size, the better. Also pay attention to the noise ratings. Coolermaster excaliburs are nice, Delta fans and vantec tornados cool amazingly well but are also hellishly loud.
CPU: The killer question. As said before, no more than 4 cores are necessary for a gaming build, and hyperthreading is a waste of effort. Higher clock speeds are better, bigger L2 and L3 cache sizes can also help. Pay careful attention here as the cache sizes quoted could be either per core or unified (all cores sharing the same cache). Also pay attention to the TDP (thermal design power) as this affects what cooler you choose. The AMD quad core FX4xxx runs much cooler than the phenom II does (and can be overclocked to about 4.5Ghz with little difficulty, I'm told), however, the phenom II x4 benchmarks higher.
Motherboard: You want one that is compatible with your CPU (Both in terms of socket, FSB and TDP). ATX boards are easier to work with than microATX/uATX. This and the processor dictate what ram you can use, and how much, so DDR3 and as high as possible mhz. Good brands are: Asus, DFI, Gigabyte, MSI. Avoid ECS, PCChips, EVGA and foxconn. Pay careful attention to the chipset; a nvidia graphics card on an ATi motherboard chipset might be what is causing the issues my mate and I have, we just haven't been able to prove it yet.
power supply: Don't skimp here. If this blows it could well take out the rest of your system. you want at least 500W. You will need PCI-E power connectors, and fewer, bigger 12V rails is preferable to more smaller ones. Choose from: Antec, thermaltake, silverstone, seasonic, OCZ, Enermax, coolermaster, corsair or XFX. Note that a higher rated PSU will NOT use more electricity than a lower rated one in the same computer; this is a common misconception. The rating is the MAXIMUM amount of power it is capable of supplying, so the higher the better, but there's no need to go overboard. Don't touch any brand I have not listed, and if your case came with a power supply then junk it unless its one of the above.
Sound card: Avoid creative, they suck. Asus Xonar are the way to go.
graphics card: Your choice is ATi radeon or Nvidia geforce. good brands are: XFX (these are the most used by gamers), MSI, gigabyte. Avoid: sparkle, evga, HIS. Recent geforce cards seem to perform much more poorly in games than their stats suggest they should, and the indications are that nvidia have abandoned the gaming sector as a target market in favour of pursuing the high performance computing sector.
Keyboard: logitech G19
mouse: I like the logitech MX518 and it's very highly rated. can be hard to find though.
plug in wired network card: if your onboard sucks (i.e realtek, via), intel is the way to go.
