Biggest factors to keep in mind when shopping for any processor:
Socket type. Later the style the better.
Speed is important, but not as important as:
The number of cores. HT or not.
Amount of cache. I mean a process that runs 3GHz with say 4mb of cache is going to get outrun by a 2.6 GHz chip with 8mb.
FSB also plays a factor, but be sure to bundle that with decent speed RAM.
Once you compare those particulars, you can then base a good purchase decision.