Is heat the only thing to worry about?

I have a 6800NU thats i'm overclocking wtih an NV silencer 5... it does pretty well, i ahve the pixel pipes and vertex shaders unlocked so far. Now if temp. is the only thing to watch out for i can OC this thing to 400/900MHZ with no issues and my temp under med-high load is only 59C. I do see a few artifacts with 3dmark05 but none playing normal games like CS:S... Just wondering if there is possibly harm being done to my card of if i'm ok to continue like this, because i'm actually noticing a performance boost here

Update: I disabled all the pixel pipes/vertex shaders and clocked it back to normal but the white specks still wont go away... really it might not be artifacts though because i only see them at certain partS (and always on the same frames) but the card could be damaged already for all i know... for the record this happens on the first demo "proxycon" or whatever mostly on the floor of the buildling when their firefighting (only when viewed from the top when viewed from the side the floor has no artifacts).

thanks

Everything seems fine on that demo... i clocked it all back to normal and playing my games fine with the temp peaking at 63C