5 box on 1 system (max graphics, 60 fps)

lenovo D10 workstation
Intel Xeon Quad Core E5420 / 2.5 GHz
6 GB RAM
nvidia GTX 470
2 monitors (22", 26")

the system can do 8 boxes at low graphics and 30 fps, at 9 boxes CPU is at 100% - so I am thinking about adding a second cpu + more ram to be able to do my 10box on one system

anyone experience with that?

my laptop can do 5 boxes on low graphics just fine (MSI GT640 with nvidia 9800)