I've OC'd my core i7 920(c0) to 4.0ghz, and it idles at around 60*C, I'm using a Zalman CNPS10X Extreme HSF, is this normal, or too high at idle?
When running prime95 under full load my cpu hits 90*C!
Should I downclock my cpu, and how long should I run prime95 to test stability at this clock?
Generally people run Prime95 at least overnight to determine if a system is stable. However, if your load temps are around 90C, you SHOULD NOT run prime 95 for more than a couple minutes until you get them in check. The i7s should not exceed 70C to be safe.
Your heatsink is not the greatest, but I don't think it's that bad either. Do you have an unusually hot room? Bad case airflow? Did you perhaps put on too much or too little thermal compound when you mounted the heatsink? Is the fan on the heatsink running ok? And what CPU core voltage are you using on your overclock?
Also, what program are you using to check your temperatures?
That's a pretty high voltage, so it could just be how your heatsink performs at those settings. You might consider reseating the heatsink if you would like to keep your settings, but I would say you are running too hot to stay there without some sort of change.