Ultra use a new revision of G80 silicon which allow for higher clocks. Maybe GTX can be overclocked to Ultra speeds, but the Ultra can reach higher speed when both are overclocked. However, GTX still have a better price/performance ratio.
Currently, the best price/performance video card is 8800GTS 320MB
Which the legendary overclocker klngpln did. 25k on 3dmark06, with quad and SLI'd 8800 Ultras, overclocked with LN. And I believe he get's them for free! http://overclockingpin.com/ultra/25k SLI ultra.jpg