My current system is:

AMD x64 Dual-core 4200+
2x 1GB ddr2 ram
7900 GeForce GS vidcard

I run 3 at 1280x1024 with high settings just fine; I ran 5 @ 1024x768(Lead char maximized to fit screen, rest in default start position on top of each other) last night and had noticeable system lag until I manually assigned cores to each instance of WoW(3 on core1, 2 on core0) and ran just fine for a couple hours, seems they shut down with less HD-fuss than my tri-boxing did too, less of an after-effect once done.

Bottom line is if yer system's up to snuff and you know how to tweak it, you can probably make 5boxing work on a "RELATIVELY" weak comp...but the suggestions above spec-wise are your best bets for high-playability lag-free 5boxing. I ordered a system similar to described above(see my last reply in my CompUSA thread in the Hardware forum for specs if you care) and can't wait to use it instead :P