I would just like to add my input. I currently own the G73 with the Radeon 5870M 1GB video card.
I've taken this laptop apart...and oh man it was a pain in the ass. Took a good solid 6 hours to take it apart, and bring it back together. I didn't even have a video at the time to guide me either (back when it was new).
I'm not too sure how your thermals will do - as well i would be worried about the power draw from a bigger video card. My G73 actually overheats quite quickly now...and i have to have a big fan on it to prevent it from shutting down while gaming.
Also I believe the GPU connector is a MXM 3 Version B in the G73 - hope this information helps you out.