I have a friend that wants to play Diablo 3 on his Mac. He wants to upgrade his video card. I am a PC guy so I am unsure of what info I need from him. I assume we will need to know what Motherboard and PSU he has currently.

This is the info I got from him so far.

MacPro 2x2.66Ghz dual core Intel Xeon
Nvidia 7300 GT 256 VRAM

What additional info do we need to get, and how can he obtain it?

In addition if we can even upgrade, what card would be good for him to use? Here are the required and recommended GPU specs from Blizzard.

NVIDIA® GeForce® 8600M GT or ATI Radeon™ HD 2600 or better

NVIDIA® GeForce® GT 330M or ATI Radeon™ HD 4670 or better

Link with full System Requirements:
Diablo III System Requirements - Battle.net Support