The benchies on Toms puts the 5770 at around 50% faster than the 8800GTS and the 5850 at around 90% faster. If you have the ping go for the 5850 IMO - it will give you the cover for the same amount of time as the 8800 has.
Odd, I got 80% from this link:

http://www.tomshardware.co.uk/chart...date-3/Sum-of-FPS-Benchmarks-Totals,1698.html
Nvidia Geforce 8800 GTS
(640 MB)
737.10
ATI Radeon HD 5770
(1024 MB)
1332.90