It appears you use onboard video! My goodness, I feel sorry for you!
Sort of going off of what Robin has posted here . . .
One thing you might simply want to do is take your system down to the computer store where you plan on purchasing your video card. They should be able to tell you what video cards are compatible with your system. From there, it is a simple manner of buying the best card within your price range. Of course, after you get a list of potentially available cards, feel free to run back here to get everyone's opinions (some people may have had specific experiences with the cards you will be considering).
Another thing you may really, REALLY want to consider is shelling out the money to have the video card installed for you at a computer store. This gives you sooo many safeguards, namely around the fact that if they break it, they buy it, instead of you footing the costs for any boo-boo's during installation (warranties and the like only seem useless until you need them!). This saved me an enormous headache years ago when the computer store broke my laptop . . . and then got to give me a brand new one (that was even better!).
Anyway, this is just my rather long-winded 2-yen.
Good luck!
~Darrin