Just built a new 5-box system based around the QX9650 and it screams (4GB DDR3-1600 / XP 64 bit / couple of MTRON solid state drives in raid0 / couple of 15K SAS drives in raid0 + a pair of 8800GTs). Managed to get it OCd to 4.1Ghz on air+TEC, but had to water cool the northbridge. In any case, it's more than responsive enough for 5 clients at max graphics settings. Got 200+ FPS on the main 1920x1200 display with all 5 on in Ironforge at prime time.

-Xeva