Well it depends how much you want to spend.
170$-200$: 256mb/512mb ATI Radeon 3850. I don't know if the 512mb is really worth the extra 30$, but I hear it helps especially if you have a bigger screen. http://www.newegg.com/Product/ProductList....=Radeon+HD+3850
250$-300$: 512mb Nvidia Geforce 8800GT. http://www.newegg.com/Product/ProductList....&name=512MB
I couldn't find any ATI X19X0 series or Nvidia 79X0 series on, but if you look around and can find one for for maybe like the 125$ price range if you want cheaper, that would be your next best bet.
I know the above probably seems expensive, but if you want to play newer games well, I wouldn't settle with less then a 256mb ATI 3850. Also, if you buy a better card now, you won't need to get a new one sooner, to play even newer games.
"I think there is a world market for maybe five computers." - Thomas Watson, Chairman of IBM, 1943