A lot of times, a simple upgrade can reap huge rewards. A single stick of RAM, a newer video card (not necessarily the leading edge one either) can make all the difference. It may not stop the decrease in FPS, but instead of 20, it may only drop 10.
Also, check to see if you have other stuff that you don't need running in the background. I have cut many things that I don't need from the services and startup menus and I get pretty decent performance.