If you are running all your characters on one PC, then I'd say a quad core is pretty essential to get a decent Frame-rate. Once you drop below 3 characters per PC Dual-core becomes perfectly viable.

You could probably save some money on the graphics card. a 512 MB 8800GTS or even a GT would probably do the job.

Take a look at Tomshardware.com to get a decent idea of FPS's and what you'd be best running with.