Alienware m17x r4 and Nvidia 880M

Hi there, i'm facing up with a big problem that really get me sad and tired.

I purchased an alienware m17x r4, insane machine and we know that. Now i would pass the kind of seller i met on Ebay. While i was opening the PC i found out that the heatsink was the one for alienware 17 r1 (2 pipes) on an ATI AMD 7970M.....(and he even insulted me about my knowledge) but nevermind. Now i think i got a damaged 7970M due to overheating issues. So here we go with my problem/question. What kind of GPU should i get? A forum member that has an nvidia 880M is selling it with his 100w heatsink and x-bracket. But i saw it's a bit difficult to install it and trying to let it working properly. Thank to @ GodlikeRU which he passed me his stable vBIOS for this card. What should i do. Buy it? In case i would, does someone know exactly the proper installation just to do not messing up somethings?

I saw an Eurocom video on youtube, seems relatively easy to install but i can not find an A12 unlocked bios and downgrade it to A11 unlocked one does not seems a good option as i read (about bricks etc). I read about a guy which installed his 880M without flashing any unlocked bios.

As you can see i have many doubts and i need a lot of opinion and suggestion. Thanks for the replies. cheers