4x slaves running at 800x600 (minimised to 640x512) with min settings on graphics capped at 15fps on my secondary monitor.
Main running at 1680x1050 with max most settings (ground clutter not included) stays at about 40-50 fps, 20 fps in main cities if not too busy.

2.4ghz 6600 core 2 duo, 3gb ram, 8800gts 640mb, 2x 250gb hdd's (no raid) with slaves on c:\ main on d:\ and slave data/cache linked to d:\
Was considering getting a second graphics card, but that would really only be to spread out the slaves, which I don't particularly need to do.