I'd try running your 4 clones on the Dual-core PC, then running your main on the lower specced PC. An X800 Pro (I have one in my box :-D) does cope quite well, even in outland for a single character. Plus the added benefit is having more screen-space for the clones.

I'd make sure you are running a 64 bit version of Vista in order to get the full benefit of all your RAM, see what its running at under load and test the systems out before commiting to buying the other accounts. That said, It *might* be beneficial to go and buy a Quad core Q6600, thereby giving you more cores to play with. Your 8800GTS should give you good FPS for the 4 clones, if they are put on reasonable settings.
Additionally I'd buy an extra cheapo 15" monitor for the box you run your main character on, just so you can browse the interweb, IRC, MSN, etc.