72c for a gpu is safe.
I am assuming you are running at the default fan speed of 36%? You should be able to overclock it and maintain the same temps if you bump up the fan speed also, but it will be louder. If you want it to be silent and overclocked, get an aftermarket. If you don't care, just use the stock cooler and bump up the fan speed.
OCing and replacing the heatsink probably removes your nice lifetime warranty. You should think about that before you OC or get a new HSF. In the case of the 8800gt, it could probably use a good new heatsink. I haven't purchased a heatsink for a GPU in a while, but the thermalright hr-03 gets a lot of good reviews. Take a looksee at that guy.
- I have one of the first 8800GT's - so its heatsink it like...not that great
- My fans are already set at 60%...
- If removing the heatsink causes me to lose warrrenty...that I dont want to do it...i rather have warrenty.