If you are going to mod a GeForce into a Quadro, keep in mind that it is most likely going to void the warranty. And if it is a expensive GeFroce ($300 to 700), then that is very painful it the power were to go off half way during the update. I don't think graphics cards don't have redundant firmware, so you couldn't go into a recovery mode.
As for the modding being capable of being done in Windows, my somewhat limited understanding of firmware tells me that if you read from a certain memory address, you can get the firmware. Once you have it, it is possible to write it to another address. Whether this would crash the OS or cause time to warp back on itself and suck the universe into a vortex, I don't know, but what I have described here can be done under Linux. In fact, I am fairly certain it can be done, since I've got manuals telling people not to write to certain addresses since it will break something.
OK, I have no idea what I just said.
Anyways, which particular model of Opteron and Athlon FX are you asking for? Generally, there shouldn't have that many differences - they are both based on the same architecture. It is just that the Opteron is mainly for SMP machines with 2, 4 or 8 processor sockets, whereas the Athlon is for machines with one processor. Unfortunately I don't have any experience with either - I'm a bit short on cash.
