I've been 5 boxing (1 pc) on Windows 7 64bit beta and love it. Great performance and stability while 5 boxing on 1 PC. Make sure you use the latest Nvidia drivers Windows 7 for the best performance. I use two wow folders which have the data, cache, interface, and account folders symlinked. Dalaran is the only place I lag which is normal for everyone in the game. I will be downloading and installing the Release Candidate of Win 7 64bit this weekend. I also have it setup in a dual boot configuration with Vista 32bit just so I can still run old games, apps, etc. that don't run well on 64bit. (only ran into a couple apps so far that don't like 64bit)

My system specs are as follows:
Intel Q9650 3ghz quad core cpu
GeForce 8800 GTX
8 gigs DDR2 ram
Windows 7 64 bit beta