you may want to consider a voltage mod. 7900GTs are actually undervolted at 1.2v. The GPU was designed to have 1.4v as the 7950GTs do.
Look up the 1.4v mod for the 7900GT. All you need is one of those pens that draw circuits.
also consider ATITool to try to overclock. The best I can get at 1.2v on my XFX 7900Gt at stock cooling is +15mhz on the gpu without artifacting.