ATI support for Windows x64 is still in "BETA" according to their site.
You need to go to their site to download the beta Windows XP Pro x64/Server 2003 x64 drivers. You cannot use ANY 32 bit drivers. At all. Not even for a joystick or a printer.
But it isn't all bad. I'm running it now, and loving every minute of it. Loads up lightning fast, stable as a rock, low resource utilization.
Works perfect with my GeForce 6600GT, SB Audigy2 ZS... also got a ton of USB stuff on there... APC BackUPS 1500VA works over USB... Logitech USB microphone works... Logitech Wingman Force 3d joystick works... any kind of generic USB thumb drives or whatnot will also work fine.
In fact so far the only things I own that are totally incompatible are my ATI TV Card (which as far as I know they will never support) and my Samsung printer (which is going to get support soon according to Samsung).
Desktop machine: 2 x Opteron 246, Asus K8N-DL, 2GB PC3200 ECC Reg., XFX GeForce 6600GT, 74gb WD Raptor, 2 x 19\" LCDs, Windows XP x64
Server machine: Intel P4 3.0GHz 2MB EM64T, ECS i865pe, 1GB PC3200, 36gb WD Raptor, Windows Server 2003
Laptop: Dell Inspiron 9100 (Intel P4 3.2GHz 1MB Prescott, i865pe, 512MB PC3200, Mobility Radeon 9700, DVD+R/DL Burner), Windows XP
Linux: P3 450Mhz, 386MB ram, Slackware 10.1 (Running mySQL/Apache)