You can upgrade your graphics card; that would be a good start. If that isn't good enough, it maybe time to move onto a new CPU, which will probably entail a new MOBO and basically a whole new rig. Your best bet right now is a new graphics card though (keep an eye on the power requirements for a card that you select).
There really isn't much that you can do, as FPS in games is usually hardware dependent. You can try cutting back in running processes (system) to maybe free up some CPU, but otherwise than that, I can't think of much that would help.