We recommend any card that supports and understands OpenGL to its full extends.
nVidia cards are well known for their excellent OpenGL support, ATI performs less in that area.
nVidia GeForce chipsets in the GTX series are very good, and not that expensive. The Quadro chipset cards from nVidia are excellent but very expensive (these are true OpenGL cards, GeForce are more gaming cards but do perform well).
Be sure to download the latest driver from the nVidia website once you plugged your new card in. Drivers are software, and software can contain bugs that are being fixed in new releases.
