You'll need a new video card (as stated by others).
It all depends on your price range. But the lowest I'd go for decent gaming is an 8800GS or 9600GT. They can be had for ~$140 and ~$150, respectively.
If you're on a tighter budget, you could opt for an 8600GT or 8600GTS. They won't be able to max out a lot of games, but they're decent cards for the price. They can be had for ~$75 and ~$100, respectively. You could also get an ATI HD3650 512MB for ~$75, as well. It performs similar to an 8600GT.