For what it's worth, here's what I'm using to 5 box on one machine. Granted, I run one a full size resolution and the other 4 at quarter-screen resolution in low detail, so bear that in mind if you want high detail and larger versions for all.

- Core2Duo e8400, it's only a dual core, but I doubt you gain much by quad core in games that don't support over 2 cores (ie: WoW). Approximately $175

- Gigabyte P965-DS3 v3.3 motherboard, older but able to support the above chip just fine, probably find something comparable for $75 or so

- 4GB of cheap RAM. Running Windows 7 RC (64bit), and having 5 copies of Wow open, I never go above 2.8GB used. So 4 is all you need unless you insist on having a million apps open in the background and watching a movie or something... $100 max, probably more like $50

- BFG 8800GT 512MB graphics card, getting a bit on the older side but still, I can keep my main window at 60fps and the child windows at 30fps pretty much anywhere now. I paid $100 for this over a year ago, so you should be able to get comprable or better for cheaper now.

That's about all that matters for running WoW, hard disks don't matter, I'm using a crappy old IDE drive and a SATA I drive. Better performance there will only affect loading screens and initial generation in massively populated areas where it needs to load lots of new textures.

I run one 21 inch widescreen monitor and a 19 inch older monitor, both LCDs though. The 21 has my main window maximized, the 19 contains four equal sized smaller WoW's. I run my main window in "Good" graphic quality, with the viewing distance cranked up to full, and the child windows at Low for everything.

I'd say it's important to run a 64bit operating system these days, for more than just WoW. It's mainly so you can take advantage of over ~3.6GB worth of memory though, 32bit versions max out around there. That's about it, if you're reusing existing parts and even if you need to buy another rmonitor, it should be cake for under 1500 bucks.

Good luck!