Man, for the price of this card, you might as well have bought a cheap i3 rig and everything would work fine. That's why you should have answered all quizzes to post on the AMD forum, of which i'm the moderator. There's a help topic there, and with that and by reading all development topics, you'd be informed that nVidia cards won't work with Mountain Lion or 64-bit mode Lion on AMD rigs because of a bug in the kernel that's so hard to solve everyone's simply moved on.
You best choice was probably to stay with Lion 10.7.5 and buy an older nVidia 5xx/4xx or an ATI 6xxx or 5xxx. If you were to insist on 10.8.x, a provisory - and very cheap - ATI 5450 would do the job until a fix comes, at least for ATI cards, which mostly work but most of them with severe glitches. Now you're stuck with a setup with which you're quite unlikely will have the full OSX experience. Sorry, dude.
All the best!