I don't know if "burning in" is useful or not, But I have personally witnessed CPUs and GPUs go under load at a certain telperature, you leave them on all night under load and return the next morning and they are a degree or two cooler thereafter.
I haven't used an nVidia GeForce card since my (very old) GeForce 2 MX-400... so out of the loop on decent updated software to tweak nV cards.
RivaTuner might let you get at the timings.
Otherwise need to try different Video BIOS(es) on the card with higher timings to permit better overclocks. (Suggest the Winfast BIOS for the exact same GPU, etc you have, WDM driver might go 'zwa ?' but still works 100% fine).
Depends how willing you are to damage (beyond repair) a AU$500 - AU$1,000 dollar video card
I'm a hi-fi audio enthusiast and it's common knowledge that circuits need to "burn in" for at least a few dozen hours before they sound the way the manufacturer intended. This applies to everything, but especially amplifiers, which produce more heat than any other kind of component. Also, all audio equipment sounds better after it's been turned on for at least 20 minutes, and it is the most pronounced with tube equipment.
Electronic components contain lots of silicon, which is a metalloid and it expands somewhat once it's reached its operational temperature. This affects the electrical properties of the whole circuit- conductivity, capacitance, inductance, resistance, etc. The first 20 or 30 hours of use for any circuit is critical in its "break-in". I'm not surprised that enthusiasts do this with their graphics cards when they first try to overclock them, and I'm sure it would make a difference, if only a small one.
If you know what type of Video RAM, and I am not just talking DDR/GDDR 1/2/3/4, etc, you are running then look up the specs of it, what other overclockers have taken that partical Video RAM to, and with what timings they did it.
Two similar GeForce 6600 GT cards may have totally different video memory installed on them, possibly both using RAM chips from Samsung, but totally different chips each with different potential and video memory ideal timings.
Heck, two 'very close revisions' of the same card may be quite different in this respect.
Can you open the card up and take a high res photo of the video ram chips ?, then someone might be able to provide some actual useful information.
lol i just pulled it out to clean the fan yesterday.. got temps down 4 degrees to 45 idle.. sigh still the hottest 6600gt out there. anywyas i dun care bout the card much any more.. im sticking ot 555/1200 untill i can afford a 7800gt. thx for all the help