Help with OC'ing 7900 GT

I have the 7900 GT with the Thermaltake Tide Water GPU cooler and switech memory sinks. I'm using NVTray with coolbits3.

I can't seem to go above
591 core
760 memory

If I try even 1 mhz higher the test fails in NVIDIA control panel, the test fails; so I can't apply any higher settings. The temperature says it's at 45 celcius. I should be able to OC much higher then right?

Is there any program that can manually OC the card like the ATI utility? The nvidia control panel won't let me get any further since you have to use their "test" option before applying.

I will probably have to increase the gpu voltage as well. How much should I limit the voltage increases by?

Go to
http://downloads.guru3d.com/download.php?det=163
here U will B able to get RivaTuner, its a utility that lets U overclock your Card manually, just be careful, once U start seeing artifacts, quickly underclock a notch or so.
-good Luck-

Like knupxfon said, rivatuner doesn't work with 7900 or x1900 series yet. I guess I'll have to stick with my crappy 591/760

Systools by techpowerup will overclock your ATI board... it is freeware too.