Why not try RIVATUNER, which has both a low level hardware overclocker or a software (hacks the nvidia driver and is basically coolbits) overclocker. The hardware overclocker actually has control over the precision of the frequency you want (for example if you want 300mhz, it can get you there as close as possible, but coolbits is often off from anywhere from 1 to 7 mhz, rivatuner is able to read the exact frequency of the chip and ram), but sometimes I think the hardware overclocker can't go as high as the software one.