I don't know what kind of software you're running, but I imagine the 3.06 w/ HT would be overkill, which isn't a bad thing at all

but that CPU will cost almost twice as much as the next fastest from Intel.
BTW, HT is not THAT big an improvement and SOME apps actually run slower because they were programed without multi-processors in mind. I think having enough RAM is most important because writes to the hd slow things down A LOT.
But in my experience, most trading apps are not very processor intensive. I mean a moving average is a pretty simple calculation.
The most important thing in my experience (which is limited) is to have stability. That's why I stay away from RAID, overclocking, cheap components. Get good RAM (Kingston, Corsaire, etc.) a good CPU (Intel--AMD's are good too but they are finicky about power supplies), good motherboard (Intel, Asus, Abit, etc..) with a good chipset (845PE/GE is very fine), good hd (IBM, Western Digital), good power supply (Antec, etc.) and you should be fine for years to come!