Core clock will affect your graphics performance primarily up until the point at which you push the limits of your graphics memory. The number of pixels you are pushing will probably be the main factor in dictating if you have enough memory. In general terms I would take the 5850 over the GTX 275 any day, but if you are utilising more than 1 x 2560 x 1600 monitor or equivalent then the extra memory might bring the GTX back on top.

Cheers