This thread is getting extremely interesting now. I totally forgot about the new i7 core but as thats too expensive yet then I understand your current choice. We have to remember people what the purpose of the machine is and what will work best together. Y2K I'd suggest you research into each component and see whats getting better reviews when it comes to performance etc. Look at the benchmarks. Apart from that as mentioned above by Dan, make sure your choice of motherboard is spot on as if its a crap board then it'll be a crap system. Its really each to their own opinion on which brand boards are better as I'm more of an Abit person myself but then again its been ages since I built a system for myself. Look into the functionality of each board and where you would stand if you wanted to upgrade.
I think the choice of a quad core cpu is good but when it comes to cpu speed its all personal preference really. You wont notice much difference between a couple of hundred Mhz so therefore you may as well just overclock but that then brings in warranty issues and if not done properly you could fry ur cpu, also if overclocking you have to take into account cooling. The list goes on and on so its kinda difficult to pinpoint on the spec you should go for.
As mentioned by many people though take into account the amount of RAM as it wont be used entirely by XP. In my personal opinion, what I think I'd do is go for a decent speed quad core cpu, nothing OTT as it may not get used to its full potential, a couple of gig ram which could then be further upgraded in the future for little cost if needed, a fast hard drive as your main one and then a big one as your storage drive. At a later date if you wanted to increase performance you could always add a raid array which would increase your data rates and considerably help performance. Hope that helps mate but as said a lot of it is personal choice/opinion really.