I'm not seeing CPUs beyond E5450 on that, and the FBDIMM memory that 2x CPU LGA771 used is very bad for gaming.
http://www.userbenchmark.com/System/HP-xw6600-Workstation/318
Aside from being slower DDR2 it has a controller on each module which adds a lot of latency. It doesn't seem to support the faster XEONs like X5470 so it's pretty limited. 2xCPU support isn't much use for gaming. Very few games will use over 4 cores. Any that do will probably choke on the speed of the CPU and and RAM. You might be better off starting with a newer workstation. The Z400 if your committed to HP, or the Dell T3500 would be better. They support 4 ,and 6 core Westemere 32nm CPUs and DDR3 RAM in 3 channel mode.
W3680,W3690 with 12-24GB DDR3 1333 Ram (3x4GB,8GB) and put the rest of your budget into the GPU.
Other CPUs are the X5687 4 core, and the X5680,X5690 6 cores. They all support Hyper Threading so they can VM as well as your older SMP machine.
You can look at the performance numbers here. The first examples of each are overclocked to 4GHz with unlocked CPUs.
http://www.userbenchmark.com/System/HP-Z400-Workstation/67
http://www.userbenchmark.com/System/Dell-Precision-WorkStation-T3500--/2522