Are you using powerstrip? Some beta versions of PS in conjuctions with certain BIOS has the problem of the gauge being reversed, where moving it up would actually underclock the card.
Try enabling coolbits or use NVMax to overclock with the built in overclocker.
There are no specific ratios, basically overclock the memory as much as you can. OCing the core makes almost no difference on a GF2 and running that higher just creates more heat.