Lower graphic options, lower resolution, close background processes, disable computer theme/visuals. That's pretty much all you can do. I'm afraid you might not get an entirely big result from it. Your FPS is based on your graphics card mostly. Your processor does the loading, and the greater your RAM the more resources crossfire can flow in.
If you want big changes, you should look into upgrading your PC. Get a much more higher end graphic card, new processor, more RAM, etc.
Answered.