I have an athlon 64 3000+ and it runs at >2ghz easily.
My board has no adjustments whatsover except for FSB, no voltage, no multpiliers, and I am using a cheap fan that I got with it for 8 bucks. It will run faster, but I am not really sure where the limits are and without having the ability to tweak voltage on anything I don't see the reason to fiddle since I won't maximize it anyway.