well your problem may be onboard graphics. something like the intel extreme integrated crud. if it's possible, replace it, even with a cheapprettythanged card you'll get far better framerates.
also, if it's possible for your PC add another stick of 512mb RAM. that'll bump up your FPS massively.
big increase in fps for a very minimal cost. in the region of 40-50 dollars depending on what you buy.