I have to agree with the above poster. My rMPB is going to be my first mac and I'm a LONG time hardcore PC enthusiast. The issues you've described are most commonly associated with ATi/AMD cards, not at ALL with nVida. This is not only my experience, but also that of the majority of real PC enthusiasts out there. Now that isn't to say there is an issue with AMD's hardware, as theirs and nvidia's hardware is rock solid (GTX 590 excluded

). The issue with ATi/AMD cards, as with your own issues, is 100% driver related and not at all to do with the actual hardware. AMD is notorious for producing horrible drivers, I've had my own nightmares with their drivers. Whereas nvidia drivers certainly have their issues, however stability is not one of them. I've ran multiple card configs for several years now on both ATi/AMD and nvidia, and hands down nvidia takes the cake in regards to driver superiority. What ever issues you had were totally driver/software related, and with the proper knowledge could have been easily remedied. (assuming your hardware was not dieing and in need of an RMA)
As for AMD or nvidia's stability in an apple product, that's something I can't tell you about as apple uses their own custom drivers. However I can assure you there is absolutely no difference between the two manufacturers' hardware in regards to their stability and reliability. Any instability issues is either due to software or hardware failure; and neither nvidia nor AMD's hardware is any more prone to hardware failure than the other. If you want to believe otherwise, your free to do so. However there is absolutely no empirical evidence to support such a claim.
I believe your good experience with ATi/AMD on PCs is mostly due to you running a single GPU, and likely NOT updating your drivers often. AMD is notorious for releasing new drivers, that only introduce new problems and fix nothing. You have to be very selective and do your research before updating your AMD drivers. While I may be overstating this a bit, but you really do have to be careful when updating your AMD drivers. Whereas with nvidia, rarely do new drivers actually add bugs instead of fixing them like AMD loves to do.
And obviously your good experience with AMD on apple machines, is well to do with apple writing superior drivers than what AMD is clearly capable of. I'm sure apple does a lot more testing than AMD before introducing updates as well. And being there are very few hardware differences between apple machines compared to the infinite possibilities on PCs, it makes writing stable drivers that much easier.
I guess my message in a nutshell, for those who are wall-o-text phobic is; GPU instability issues are always due to software(drivers) or simply a card that needs to be RMA'd (hardware failure). Neither nvidia nor AMD have shown to be more reliable than the other, however nvidia tends to build better reference coolers. Being that your concern is with AMD vs. nvidia on an apple machine, the difference in hardware is only going to be a difference in performance not in stability or reliability. It's up to apple to make sure the drivers are rock solid, and the cooling system adequate to maintain long term reliability. So if there is a difference in stability between them, it's quite safe to say it's apple's fault.
I know this isn't exactly the most technical forum, so I do apologize for getting over technical and long winded, however I don't know how to post any other way

.