Quad cores overclockes not so nice as dual cores, you can get something in range of 3-3,4GHz. Test in Prime95 10 hours or more to get fully stable mode.

I have 20" monitor with 8800GTX factory overclocked and perfomance not so nice, so GTX280 will be better, new games wanted even more powerfull cards
After upgrade from 7950 to GTX280 you will get very nice perfomance improvement, about 3-4 times faster.