iMac 21.5″ (EMC 2428) Teardown � iFixit Blog:

With a bit of magic, the GPU heat sink detaches from the logic board, exposing the AMD GPU board. You heard that right, folks — you don’t have to replace the entire logic board if your GPU explodes from too much l33t gaming. You can just swap out the GPU board for another one.

If you're just joining us, the original iMac and a few versions after had a "mezzanine" slot that allowed some folk to shove a Voodoo 2 video card into the beasts. I've always thought that was pretty cool, if horribly impractical.

Looks like that slot's back though, honestly, it'd almost be better to go the ASUS Game Station route and slap it into the Thunderbolt port.

Wonder if coolness will strike twice?