You should be able to
OC it some with the stock cooler, just run 3DMark01SE, 03 ot 05 after each 5mhz or so. If you see any artifacts (things that should NOT be there, white/black dots and tears are the most common) then you have pushed it too far.
Don't worry about killing your card via overheating, to do so it needs both more voltage (volt-mod) and to be run at frequencies that artifact in Windows for quite some time. Hell, I think all modern video cards will automatically turn off or scale down till the temp is safe.
To the get the max
OC you will need better cooling though, Arctic Cooling (
VGA Silencer), Zalman and using
CPU heatsinks (requiers modding) are your best choices. Obviously water cooling or phase change cooling will be the best but they are not as practical.