thasan1 :
GF GTX 670 performs close to it
Thats where you're wrong, the 580 slots in between the 660 and 660Ti, i used to own a 580 and then upgraded to a 670, i can tell you the 670 is much more powerful
@OP: with your current 7850 PE, its overclocked so it should perform similarly maybe a bit slower than a 580, but then you can also overclock the 580, if it were me and if my PSU can deliver enough power, id go for the exchange, but the VS 550 is an entry PSU from corsair, though it will be able to power the 580, you will be at the limit of the PSU