Well the iMac currently has two different cards:
-2400 xt
-2600 pro
The Mac Pro's 2600 xt is roughly the same as the 2600 pro, the GPU clock and memory speeds are different but not THAT different. Don't get the 2400 xt if you plan on doing any gaming. It cuts down on the number of shader units and games will suffer.
If you're not doing any gaming, all three cards are about the same in terms of picture quality and 2D graphics.
Actually, Im going to purchase a Mac Pro. Im just trying to justify or not the 8800gt. I have messed around with an imac and the performance seemed adequate. I already have a pretty good screen and with the lack of expandability in the imac is probably not where Im going.
The other day at the Apple store I was playing around with two Mac Pros, one was a 3.2 with the 8800GT and the other was a 2.8 with the 2600. Everything else was the same. The 3.2 was very noticeably quicker and more so than the 15% CPU speed increase would suggest. Could the difference in the GPU be adding to what seemed just like a snapper computer or was that just my imagination?
@bluedoggiant:
Good explanation but how is that different from what I said?
Desktop XT:
http://guru3d.com/article/Videocards/440/3
Mobility XT:
http://www.notebookcheck.net/ATI-Mobility-Radeon-HD-2600-XT.3770.0.html
Same number of pipelines, different clock speeds. Basically the same...