It is not clear if your frame rates are limited by the vga card, or the CPU. It's probably a bit of both, depending on the game. I suspect it's mostly the gpu.
As a strategy, upgrade the video card to a high end single card like the GTX280 or 4870X2. This assumes that your upgraded PSU has two pci-e connectors, which indicates enough power to drive them.
If the improvement is not enough, then it is time to replace the cpu part of the system, keeping the new GTX280/4870X2