I was running four clients on my pc last night. AMD X2 5200+ overclocked to 3GHz, 4Gb memory, and 8800 GTX 786mb (or whatever) on a stripped down version of XP SP2 x86 (most services/excess stuff disabled or uninstalled). I'm running EQ2 of a 5 SAS Drive Raid5 set. Using innerspace (or another app) to lock down the FPS to 20 on the 'inactive' windows helps alot. I also run the 3 inactive windows at 'very high performance' settings and didn't have any problems running my newbie team through the RoK newbie area. There is a bit of lag at times when switching between windows, but it's very minor. I also noticed that each client takes ~ 500 mb of physical memory to run, so If you have less than 3GB of memory (or PAE is disabled), then you're going to have issues.

Once i get a few things figured out, i'm going to shift everything over to my 4x4 and try 6 boxing on 1 machine... Should be interesting.