are you worried about power consumption.?
the GTX 570 is better in performance, both cards can still be overclocked with programs like MSi Afterburner.
the HD 7850 is more power efficient.
if you have the ample power supply and the pennies it costs more to run the GTX 570 doesn't bother you.
(and I did say pennies more a month)
I'd go with the 570 but it's a preference I have.
look for benches in the games you run and resolution in particular..!
with that being said, when does the GTX 660 and GTX 660-Ti release.?
i only have a budget of £850 and i have already got the case and the CPU i would not be able to afford the ample PSU and originally i was going to go for a OZC 850watt 80plus gold psu