Currently 5-boxing with one 21" monitor (soon to be two) on the following setup:

Intel Core 2 Duo E6600 (standard 2.4GHz clock atm)
4GB RAM (only using 2.75 under XP)
Asus EN8800GT video card w/512MB

4x alts at 800x600
1x main at 1280x1024

All windows are at 60 FPS until you get into crowded areas. Then they drop to 45-ish. Very playable and I highly recommend the 8800 GT if you are speccing out a system right now, especially considering that the WotLK expansion will bump up the graphics.