Surprisingly my comp isn't all that hot:

AMD 4500+ x64 dual-core processor
2gb DDR2 dual-channel ram
IDE HD that's so old I'd probably be ashamed to post it's specs if I knew what they were(Though I have a brand spankin new(or was new when I bought it last year LOL) SATA HD to hook up eventually)
7900 GeForce GS(I think GS, was bought new sometime last year pre-8800s)

Fairly simple, possibly a lil old/weak compared to the more monster systems other boxers use, but mine runs 3 1280x1024 maximized/windowed WoWs without turning down graphics settings on ANY of them just fine. I run a minimalist setup though, the only things besides windows shit I have open are obviously an AV/firewall, and mIRC. Lately when I multibox I'll start Windows Media Player up prior to WoW cuz it does seem to help slightly(that was no myth but your results may vary); leaves more CPU open for WoW, turn off all your extra shit, you don't need torrents or too many messengers open, only pop firefox open when you need to(and god help you if you still use IE, I'm a stubborn fucker afraid of change but even I use firefox), close down ANYTHING you can even if it's just idling or seems to take nothing to run.