I have a Mac Pro 4,1 with an NVIDIA GeForce GT 120. It can't handle detail intensive graphic without slowing down and/or crashing programs. I need to upgrade. What should I get?

You will need to get a new computer. You cannot upgrade the GPU because it is not replaceable.

EDIT:

My apologies. I thought I read MacBook Pro. Stupid me. You can replace the card with one of those available from Apple like the ATI 5770 or 5870. Both would provide a significant improvement in framerates and performance over the card you are currently using.

A graphics card that is working properly may not give you the frame rates you most desire, but it does not do what you describe. You have other problems, or perhaps the card is broken.

If you simply want to replace it, the Apple/ATI 5770 or 5870 are about the only candidates at this writing. PC versions of those cards will not show a picture under Mac OS X, and flashed cards will not show a picture until Mac OS X is fully operational, which means they are useless for debugging when things go wrong.

Graphic card comparison Mac Pro

Graphic card compatibility

Graphics Cards Apple Store

Nothing in there that says "No ATI 5x70 for you" and it is a very common popular upgrade from GT 120, ATI 2600, and others.

Radeon 5870 vs 5770 GTX 285 4870

All the way back to 2006 too. 5770 in the 2006 Mac Pro

There is no tech note: ATI Radeon HD 5770 Mac Pro pre-2010

Comments from buyers: ATI Radeon HD 5770 MC742ZM A