If you want a dual core cpu then get the X2 3800+, I have been reading around ocuk and it seems all the latest X2 3800+ chips are hitting around 2.6 - 2.8ghz at good volts.
If you want to stay single go for the Opteron 146 early steppings e.g 0540 or the latest steppings as both these clock very well with pretty much stock volts.
I have a Opteron 146 CABYE 0540 FPAW that does 2.9ghz on stock volts
The choice is yours...
Hope this helps, Sam