So I recently got a Galaxy GT 640 CPU, and 1gb of DDR2 1066 Corsair Dominator RAM. I had the same amount of ram before, but it wasn't as fast. I can play some games but still at a high CPU cost. Other games won't even load. My PSU is a stock 300w PSU.
To put it simply, your computer is getting a bit old. I think you have a pentium D cpu in that, newer games a really going to stress it out. The powersupply makes no difference. Also, the amount of RAM you have is a little low. The only way you're really going to solve performance issues is with a new CPU, motherboard, RAM, and probably powersupply. The GPU you got isn't top of the line, but it should be able to run just about any game, some on low settings and resolution.