that gfx card is already plenty powerful. the next step up from a 5850 would be a 5870 which would cost another $100, so unless youre sure your uncle is willing to drop some serious money on it, a 5850 should be more than enough for you.
also, no you dont need the black edition. you can acheive the OC that the card comes with yourself so you (or your uncle) should just save money, its not worth the extra $60.