This is no longer true. As of the last revision, it's a desktop GPU.
This is not true. The graphics card in the iMac is a mobility card:
https://forums.macrumors.com/threads/978676/
The iMac can game decently, but no where even close to the graphics card in the mac pro, which is the 2nd best ATI card, behind the 5970.
I am by no means knocking the iMac's performance, it just doesn't have the power of the Mac Pro. The 6 quad mac pro, with the 5870 can go head to head with most alienware setups and other gaming rigs...that is if you are gaming on the windows side 😀