DirectX [as high as possible on the hardware video card] is better. Videocard has to be as insanely good as the rest of the system of course Remember, hardware accelleration is the key here. Every card can emulate stuff via software or their GPU. The main thing to watch is what all the hardware on the video card can.
-Expanded memory from 3.2G to beyond or staying at 3.2G?
If 32 bit go with 4 GB (3GB is addressed, but just use 2x2 for stability). If 64 bit go for 16GB or something.
-Will your O/S be still support for updates when M$ stop supporting it?
Doesn't matter for games.
-How many people using your computer?
Does matter only when you use the system as a multiplayer host, otherwise just don't do it.
Better security in x64? I think but I have to check more into this part......
Not applicable for gaming. Perhaps only for some online gaming.
And if you're already investing lots of $$$ in this monster, better get 2 x SATA II 15k rpm 16mb cache disks in a RAID as well for extremely high performance. All hardware accellerated on the motherboard of course. The OS of course runs on a seperate SATA II 15k rpm disk