Bios compatibility on many main boards and ram.
I ran into compatibility problems which required 8 trips to Frys before I got my money back and bought a board from another vendor. ASUS, Gigabit and ABIT had these problems. I bought an ASRock board and Kingston RAM and had no further hardware problems. However the Microsoft 64 bit Windows beta was very buggy, but the latest update works great. I love the raw speed of the true 64 bit executions.