OK, first, I do not think a OC'ed 7600GT will compete with a 7900GS. Ever. But OC'ing definitely gets closer than stock speeds. Your OC is safe now, but try to see if you can get that core up to 700 without crashing. And by doing some reading, don't expect to go much farther than 700/800 without a volt mod.
Now for coolbits, it is simply a regestry entry that allows OC'ing of a nvidia video card. The driver fully supports it, its just locked to begin with. Just Google it, and there is a file you can download and run to use.
Hope this helps.