Those who say that raising vcore on 7800GT not is possible without hardmod are wrong. Havn´t tried on GTX, but i think it will work on them also.

Open your bios with nbitor then change device to 6800series.
Save bios then open it upp again. Now you can use the extended voltage table. After changing to 1,5, change device back to 7800gt. Save.
Flash your card and mesure if you don´t belive me.