I don't know why you've got such a high temp here... I used to have a 6000+, using the same cooler i have now, a freezer pro 64.
It used to idle at 24-30, and never went above 45-50 under load.
Agree with the others about it not being a very good overclocker. I used to mess with 'FSB' and lower the multiplier to get it to 3.2GHz, and had my GeIL PC2-6400 Ultra running at 880Mhz at 4-4-4-12 @ 2.1v.
It's also strange that you can't lower the voltage.... There's a commonly known problem with the M3A78 Pro that doesn't let you increase the voltage, but i've never heard about not being able to lower it.
Is it a recently bought chip/motherboard? It may be that one or the other is faulty?