Welcome to the forum. What we need to know is what the rest of the system is like. These types of systems tend to be 3.0 GHZ CPU's with 1 or 2 GIG of RAM, and tend to come with a 17 inch Monitor. If that is about the same as your system then i would sugest a HD4670. They are relativly cheap and powerful and dont need any extra power from the psu.
Let us know about your system and we can go from there.