It's a slightly older iMac with the nVidia 7600 in it.
But honestly - I don't think it matters which iMac it is (other than the video card is causing problems).... the problem is the video card firmware update isn't compatible with Lion. I'm sure if I was on 10.6 this would all be moot.
According to http://support.apple.com/kb/HT3986 inorder to install Win7 on machines with the listed nVidia cards (which mine is one of them), you have to install the firmware upgrade for the card... which of course I can't do because I'm running lion.
A vicious circle.
All of that... is the "grain of salt" and that is all.
Installing Windows 64-bit isn't supported but even that people have managed to overcome. And it matters as to how much RAM you can have, and processor (and there are some models that if you want people have swapped in newer processor of same family).
And if you want 10.6 (and I doubt you need the firmware update as a deal breaker) install SL on another drive! or why didn't you clone your system and keep 10.6 around for awhile when you upgraded to 10.7?
It is not catch-22.