Problens overclocking 7900gs

my eVGA 7900gs runs at 450/660 and from what I've hear they can easily be overclocked to around 550/700. I have tried and it works at that speed, no artifacts in 3dmark 07, or in my games. It only gets to about 57C under load at those speeds. anyway I decided to play it safe I set the clock down to 500/680, but regardless of the overclock it resets to stock frequencies whenever I restart. I am using nvidia ntune for overclocking, all I have changed is the GPU, my computer is a compaq and thus not capable of overclocking the RAM or CPU.
My specs
600w Ultra power supply
ASUS amberine M mobo with ATI xpress 200 chipset
2gb DDR RAM
Athlon 64 3500+(socket 939)
Windows XP home.

 

You can use Riva Tuner to OC your VC, you can choose to save an OC setting.

I think you can do the same with ATI tool

ATi tool is very good for checking artifacting