This could be because of a lot of things bud.
Drivers are definitely one of the things that you want to look into. Make sure that your drivers are up to date, and that you have any patches for the games that you play (steam should do that for you).
The next step would be to look at using a benchmark utility. By using one, you can find out why your computer isn't living up to it's word.
I suggest viewing this movie from the show, "systm" at revision3.com to help you with just that.
http://revision3.com/systm/benchmark/