4GB is plenty enough to 5-box. We used to do that on Q6600 quad cores before 64bit Windows and the i7 CPUs became popular. The biggest problems are the CPU and graphics. 9500s are mainstream GPUs. I suspect you can still multibox 4 WoW clients at super-low settings on all 4 and a minimal addon setup. For the newer games, I think you'll be hard pressed to run more than 2, maybe 3.