Not to be rude, but if you get considerably low FPS, it shows that your graphic card is either unable to handle the game, your computer is overheating (throttles gameplay like a bitch), or you don't have enough RAM.
If your graphic card can't handle it, you'll have to buy a better one. There are some good ones priced at under $100, but make sure its compatable to your computer (check manufacturer and model).
If your computer is overheating, clean the fans with a compressed air can. Make sure the computer is off before spraying it though, and only spray in short bursts... Air cans are cheap at like 2 bucks.
If you don't have enough RAM, add 1gig more, if you buy RAM online its as cheap as 25 bucks per gig. Also, make sure the RAM is compatible to your computer and model as well.