memory requirements (all numbers are rough, obviously):

OS - xp(500M) vista64(750M)
wow - 500M per instance

5x wow + vista64 == 2.5G + 750M == 3.25G

keep in mind, to avoid memory thrashing you need to be BELOW 80% memory usage. therefore:

M * 0.80 = 3.25
M == 4.06 G

so, you need at LEAST 4G of memory to run without thrashing while 5 boxing. keep in mind, as you move around the wow world, each wow instance increases its memory usage (a little, they have definitely improved on this). also, if you have a browser open, maybe winamp and an IM client, you'd want even more memory.

6G on vista64 would be a very nice fit (i'm running with 8G on my vista64 quad core at the moment)

as for issues with win7 ... i haven't loaded it on any machines yet.

drop me a PM with your email address and i'll check to see if there are any issues.