Hi yes you should be able to overclock the GPU a little, I'd prefer to use RivaTuner rather than NTune though.... its been a long time since I had to OC Nvidia gfx cards, but I beleive that Riva Tuner is still the preferred tool for OC'ng the NV Cards...!!
Try googling "RivaTuner OC Guide" or simialr and you should find plenty of topics to get you started....
RivaTuner Overclock Guides
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3DMark06 =
20717 / 3DMark Vantage =
P13618 /
My Overclock Guide /
My Troubleshooting Guide