The ATI 4870 should have OpenGL 2.1 support.

Personally I would not get the nVIDIA card UNLESS the drivers were specifically written by nVIDIA and not Apple! Matter of fact, I would never buy another nVIDIA card for a mac again. Apple writes the worst drivers (in conjunction with the manufacturers) for their cards. I settled for a nVIDIA 8800GT card when I got my mac and I might as well have gotten the ATI 2600. I've had screen tearing issues, lag issues and kernel panics due to the driver. Card works fine in Windows XP and Fedora & Ubuntu.

If you have Photoshop CS 3 try and pan across a large image. The whole image tears up and goes all blocky! Playback in Shake is near useless as the screen tears in half with ANY playback, even scrubbing doesn't work. ( and this happens on two monitors with clean installs)

I vote save your money and keep the ATI.

What issues are you having with C4D?

-J