Hmm.... a GTS250 is a 9800GTX+ right ? But I guess that too overclocks a lot.HD4770 OC
Core - 850MHz(gos to 970MHz actually)
Memory - 1000MHz(goes to 1150MHz actually)
This setting beats the HD4850 by more than 7% and matches or beats the GTS250.
And HD4850 can be OCed:
Core - 700MHz(from 625MHz)
Memory - 1100MHz(varies, max 1150MHz)
But at this setting, the card JUST would beat an OCed HD4770.