2.8 to 3Ghz on Stock cooling will be perfectly Fine, I did that myself, you can reduce the voltage to keep it cooler at those speeds. Unfortunately, it is not stable below 1.32V at 3.2Ghz.
Thats a shame as my mates 6400 runs at 1.2375V at 3.2Ghz, so it runs a lot cooler than the . Of course the CPU cost him more.
Also, I've given up on the Intel Thermal Analysis stress test, I've gone back to prime 95 / OCCT which produce core temperatures far far lower than the TAT tool, and seem more realistic. ie, if it's stable with OCCT / Prime it seems to be stable in use.
Have also gone for water cooling and am now at 3.339Ghz @ 1.41V with a core temperature of 59 degrees. I'm going to get some better memory and try for a bit more.
I have the F6 Bios F/W, which is what it had on it when I bought it - booted up straight away with the Core2 in it.