Computer runs some games slow and others fast?

My computer just today has started running some games poorly and others really fast. I have a Nvidia GTX 550 and 16 GB ram running windows 7 64 bit.

I can run GTA IV on the max settings with no issues at all, but then when I try to run Episodes from Liberty City, it lags tons and I cant alter the graphics settings at all or even switch up the resolution from the lowest setting because I apparently dont have enough video memory. On GTA IV's settings screen it says I have 900 and something video memory to use and Episodes say I only have 54.

Its a similar deal with Arma 2. Arma 2 runs awesome on high detail, but Arma 2 Operation Arrowhead barely runs on the lowest settings. What the hell? Its as if my PC isnt even attempting to run these games as well as it clearly can.

Yep just tried that, got the latest one but it hasnt done anything.

Thing is, GTA IV is set to max, and the resolution is 1900x1080, and Episodes wont even let me change it to anything from 800x600, because I apparently dont have enough "resource usage".

Could it be that it just didnt download properly from Steam or something? Seems abit odd it would happen to two games though.

Tried redownloading it, no luck. Like I said, its as if certain games are just refusing to recognize the hardware. Also I'm not sure if its a coincidence or not, but games downloaded prior to yesterday are the ones that work fine. Any downloaded after that are the ones messing up. Though when I uninstall the working games then redownload and install, they still work fine. So I have no idea whats going on. Ive tried uninstalling every game, uninstalling Steam, then reinstalling everything and theyre still shitty.

I tried that, nothing.

The computer recognises the Graphics card, it comes up under device manager and everything but when I play a game or something it just doesnt even attempt to use the graphics card, it says I have 0MB video memory.