um.... anything really.
If you do intensely detailed rendering, then get a higher-end card. The more memory it has, the better. I like to stick with nVidia cards (they've never failed me yet, but all the ATI cards i've used except one all overheated)
So let's see... i would recommend a midrange or higher nVidia 7-series card.
Like I said, the more detailed the models, the more power you'll need.