This is an extremly ATi Biased statement:
Im confident that incresing your memory clock will not give you more Mhash/sec but instead heatup your card and draw more power.
Power=Free (for you)
Card=Free (you already own it)
The heat blowing out your case=Free (but unwanted)
I honestly think that if you intend to Mine coins, Sell your nVidia GPU and buy an ATi GPU, They are the KINGS of mininghttps://en.bitcoin.it/wiki/Mining_hardware_comparison
Like Damn dude a $200 5770 lays the fucking SMACK DOWN ona $600 GTX580
They both get avrg of 150Megahashes at stock, and i have a 5770 pushing 195Megahashes when it's oc'd