I paid £169 for my P4 3.2Ghz Norwood ($316) at a computer fair. It was a bit cheaper than the AMD64 3.2Ghz.
For Barret it depends on your money situation. If you are likely to be able to plough more money into your system in the near future then by all means go for the higher chip.
The higher the chip, the more you can overclock it (on most occasions).
As long as you will still be able to afford the water cooling etc sometime soon.
If you wont have cash at you desposal then, by getting say a 3Ghz chip, using the cash to purchase a watercooling system. Your machine coud maybe run at 3.6Ghz or even higher.
But be warned, If you get the overclocking bug it can be VERY Costly.
If you go to the www.custompc.co.uk
site and look at the overclocker there,you'll see what I mean. My machine starts to look like Camers (sorry!)