I've been running XP-64 for more than a year now on a Dell Precision T3500. It's a beautiful thing: 6GB of RAM, (4) drive RAID-10 system, nVidia graphics, dual screen. Everything works: printers, scanners, mice, sound, video. Oh, not quite everything: the only thing I've found that won't work is iTunes.
Still, I'm giving serious consideration to migrating to Windows 7 64-bit. Once I try it on another machine, I'll use the new VMware 7 migration facility to wrap up my nicely customized box as a virtual machine, add another 6GB of RAM, and install Windows 7.
Any caveats on the VMware Workstation 7 migration tool?