It been confirmed that you can not unlock it with RivaTuner. But there may be a way to unlock it with a bios Flash.
IMHO the Gt is defiantly the winner. Its only 5 10 fps slower than a GTX in most games, and is 100-140 bucks cheaper. With that extra cash you save, either go buy more games, or get more Ram for all the recent Ram munching games.
AMD X2 555 unlocked to X4 B55 @3.5 | 8GB GSkill Sniper series | Asus M4A89GTD Pro/Usb3 | 2x XFX HD6850 | 600W OCZ ModXStream | Thermaltake V9 Black edition | Corsair H50 | 2x 1TB Samsung HD's