the 300/1000 is the 2d speed. Don't change it! There is no reason to have it running at 70-80C all the time! Go to the performance 3d mode, then over clock from there. My suggestion? Go to "detect optimal frequencies" and then lower the results by about 7-10MHz core and 10MHz mem, then try from there. For example, my optimal freq was 579/1150, so i went to 570/1140 and it's stable. I recommend running HL2 and looking for artifacts, since that runs very hot and is good at generating artifacts if it's not stable. As for OCing, I've never heard of a 6600GT hitting 1300MHz RAM, unless you have the XFX "Extreme Gamer Edition" that uses 1.6ns GDDR3 (1200MHz) default. Since youre 2d is 300/1000, I doubt you have this card, so you should probably tone the mem speed down, or you'll burn out your chips.
As for good OC results, if you have kind of a crappy cooler (XFX's stock sucks), 545/1090 is reasonable. With good cooling, 575/1150 might be expected, with some very good cards hitting close to 600/1200. No matter what, 1300MHz RAM isn't gonna fly for very long.