Your RAM and your video card are your biggest bottlenecks, in that order.
Upgrade to 4GB of RAM ( 3.x if you are using a 32 bit OS ), and look for a video card in the $100-$150 price range and you should see a noticeable increase in your framerates.
Your HD is also getting long in the tooth, however, most games load all the key level data into memory first and access the files from there. You will want to replace this also soon, but I would look at the other areas first.
thanks, your suggestion seems it should have been the best answer.wasn't sure if inactive ment nobody could post anything,but yeah i was second thinking the HD was the main problem too.like you say, memory and GPU seem more likely.