One of these perhaps would be a reasonable start click here its in your price range but the graphics isn't great. I have a 7600GS and so far its played everything at a reasonable frame rate but I keep the settings to 1024x768 and I have in a few games dropped the graphics options down a tad. They are top end budget cards rather than gamer cards a 7600GT would be a bit better but the 8800GTS seems to be the card of the moment but you might be struggling to get one within your budget. ( mind you, you could upgrade later if necessary).
These are better obviously hence dearer but worth considering click here
If you can use your old monitor then fine, if not you can get a new 19" TFT for about £110 mark. click here click here