Couple of great articles showing how far these boys can be pushed and the resulting performance increases, on the 275 the individual shader, core and mem overclocks didn't make a huge improvement on their own but when combined gave GRID and COD4 a 30% performance boost, damn!
Nvidia 275 - http://www.anandtech.com/video/showdoc.aspx?i=3575
Ati 4890 - http://www.anandtech.com/video/showdoc.aspx?i=3555
Nvidia 275 - http://www.anandtech.com/video/showdoc.aspx?i=3575
Ati 4890 - http://www.anandtech.com/video/showdoc.aspx?i=3555