Upgradable iMac Graphics Card?

Okay The new MacBookPros came out today and three of the four models sport the NVIDIA GeForce 8600M GT 512MB graphics card. and I say card because these reportedly use the NVIDIA MXM graphics card connector. I remember a while back that I read in a Gizmodo article that the new 24″ iMacs used the MXM connector and were theoretically upgradeable. Do you see where I’m going with this?
Without going the MacPro route, Mac users have been pretty much stuck with whatever graphics card that came with their Macs. For some people (read me) Mac graphics cards leave them with PC envy. Especially if your into online competitive gaming, like Second Life formula car racing, for example.
This is a stretch, so try to hang with me. Since I’ve got a 20″ iMac with the 256MB Radeon HD2600 in it and 24″ iMacs have the same graphics, it would make sense that both have a MXM connector. So If I get my hands on a 512MB GeForce 8600M GT which also uses the MXM connector and is used in a Mac and therefore must have Apple drivers, I just might be able to double my graphics power (and keep up with PC Racers Muwahaha! )