unless you are adding voltage (in this case you wont be) the temperature increase wouldn't be significant in my opinion, so i dont see a problem with overclocking the graphics.
there is no definate number that you can overclock your card, just enable coolbits (search for coolbits 2) or download rivatuner, overclock the card until it fails test. im not sure if it makes a difference in this case but my 6600gt is running fine at 570 Mhz core and 1150 Mhz Ram. if you use the coolbits option it will find the right settings automatically (albeit very conservative, you can push it a few more MHz). rivatuner will is a bit more time consuming but still very useful.