I ordered the card from weloveMacs.com. I had the nvidia 7300 with only 128MB memory. I ordered & installed the NVIDIA GeForce 7600 GT VRAM 256MB Video card for $299. It was about $338. Withbexpedited shipping. I did take some pictures that I could send you with comments if you like. I had replaced the HD on this machine before. That was fairly easy. Everything I read about this video card swap made it sound particularly hard and daunting. It really was not. Yes, the profiler recognized it immediately. My screen is brighter and my games are much faster too. The whole machine seems faster.
Yes it is possible. I just replaced my ailing 7300 with a new 7600 and it works great! The card and it's thermal channels are at the front of the machine, right under the motherboard, so it is a complicated repair, but well work the labor. I saved about $500. doing it myself.