Looking at the prices on NewEgg, it seems you can get an ATI x1600 for a little over $100, or a similar NVidia card, the 7600GS for around the same price. Previous generation is going to be the x700/x800 or the 6600/6800 line of cards.

The 75-100 range seems to be filled with some cards you will want to avoid. Do not buy any NVidia 5xxx product, and stay away from the even older MX series. It looks like the NVidia 6600 sits in this price range, and should be good enough for what you want to play. ATI wise, it seems a lot of the cards in this price range are 9550 or 9600 cards. They would be an upgrade, but not a huge one, and considering the price of the cards above aren't that much more, it might be worth it to spend slightly more.

My concern with any of them though is the need for the extra power. I did run an ATI Radeon 9700 Pro in the SS51G for a while (and my friend who bought the system still has it running), so at long as these new cards don't need all that much more power, it should be fine.