Well... I might be able to help here, because we've just upgraded our other machine to a P4 3.2D 830 (Dual core) and we have been testing it against my 2.8 dual xeon nocona...

The dual xeon destroys the 3.2 dual core in every processor benchmark test... (It does have 4gb ram compared to the 3.2's 2gb but on the processor test memory isnt touched)

In the real world IE rendering timelines etc the dual nocona wins again hands down...

With the xeons being hyperthreading they show up as 4 Processors!!! its beautiful watching them all working in task manager when exporting etc.

So For pure processing power Id go for the xeons, but the mobo will be slightly more expensive, also theres only a few xeon mobos with AGP I have the asus ncch-dl which is wicked...

Also dont forget to budget for a 24 pin SSI powersupply... (Mine was £165 jesus was that a shock after forking out for the rest of it! ) but have come down now around £70 I think)

I hope that helps, feel free to ask if you want any more info.