Forget the PSU - the graphics card is disproportionately powerful compared to your processor. For gaming purposes you should seriously look at a system upgrade as a whole rather than a new GPU. While daunting, you're not going to see the performance increase from a GTX770 that you should merely because of the slower processor.
That said, I wouldn't run the card on an older PSU like that. The 770 is a beast and might prove to be a bit much.