I'm sure 2 3870's would be more than enough, just one compares close enough to the 8800gt.

I have an 8800 GTS 512 (had an ATI 3850 but shelved it due to poor game/driver/vista support) and use a program called Keyclone to control the frame rates of each WoW instance along with being able to assign each instance to one of the 4 processors in the Q6600 (this really helps when multi-boxing on one comp). The only issues I have 5 boxing is when I enter Stormwind, I have to give it a few seconds to load on all 5. Other than that it runs great.

Keep in mind, Windows Vista 32 only supports 3 gigs (actually 4 gigs but that also includes any memory your graphics card comes with, so you will only ever see/use 3.? gigs) of Ram and Vista 64 bit supports up to 8gigs. 3 gigs should be more than enough even on a 64 bit system unless your into huge video edititing or graphics design/rendering.

If your buying a new computer, I recommend doing research on a complete clone of your hard drive while it's still fresh. That way you will be able to "ReDo" your computer when things start to slow down, you know, when you start filling your computer with useless crap. This will affect multi boxing on one comp.

Also, if your building a new comp try and get a SATA II hard drive, they are super cheap, and the difference is huge.

Hope that helps.

EDIT: Almost forgot..... Welcome to the forums!!!