Your mileage may vary, but the cookie cuter 5-box single computer is something along the lines of:

Intel quad core Q6600
4 GBs memory, either 800 MHz or 1066 MHz
Nvidia 8800 GT

Those components should be relatively inexpensive at this time. That will give you 60 FPS on your main window with most video settings to high and about 30 FPS on your other windows with most video settings to low.

Also take into account that you need a 64 bit OS to fully utilize the 4 GBs of RAM.