IA-64 refers to the Itanium's instruction set. What you meant to say was x86-64, which is for the Opteron and Athlon64.
Unreal Tournament 2003 has already been demonstrated to run on the Opteron with nVidia graphics chips, so drivers do exist at least for the video. Theoretically, at least the sound will also work under "64-bit" mode. Since the LAN driver is binary, it's a crapshoot if it'll work or not. It probably will, but if it doesn't, you're not going to be able to fix it.
A word of warning: the Athlon64 is going to change packaging (right now, it's the same number of pins as the Opteron) to have fewer pins in a few months after release, so I'd recommend waiting until AFTER that to buy up.
AFAIK, both SuSE and RedHat will have distros ready for general use with the release of the Athlon64. You could also try Gentoo if you're feeling brave.