Average Overclocks of ATI HD 4890 and NVIDIA Geforce GTX 275 from Reviews
 

Interesting :)

for previous generation, you can get OC numbers from the massive database with thousands of OC results at HWbot
NVIDIA http://hwbot.org/browseHardwareVideo...puFamilyId=162
ATI http://hwbot.org/browseHardwareVideo...puFamilyId=179

8800 GTX OC vs HD 4870 OC http://hwbot.org/hardware.compare.do...43_1&id=1361_1