I think it's been made clear that the PCI-E is a given, but good luck on finding a barton X2.
Current cores on sale for X2s are Toledo and Manchester, with the later being the most recent revision. (Although the Manchester is actually a cost cutting revision)
For a price guide (Retail), you are looking at ~£620 for a top spec X2, and ~£240 for a bottom spec X2. The 3500 the folks above have suggested drops in at around£140-150 and will give you the most bang for your buck at this moment in time.
The X2s excel at multithreaded applications, so unless you have something very specific that is intensive in a multi CPU way I'd recommend the same as above.
With some of the money you'll save from not buying a silly priced X2, I'd also suggest a 74GB WD Raptor (10KRPM SATA) for program storage, and a 250GB SATA drive for storage (or 2 if you can afford them and think you might want to develop a video collection
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). The reason for this is windows can swap to disk faster, read, write and load stuff faster from the raptor. 250GB is the minimum you should consider HDD wise if you want to get the most for your money.
80GB * 2 @ £41 per drive (£82) = 1.9GB per £
120GB @ £58 = 2.06GB per £
160GB @ £54 = 2.9GB per £
200GB @ £60 = 3.3GB per £
250GB @ £70 = 3.57GB per £
300GB @ £87 = 3.4GB per £
Defrag time is not too bad as long as you keep it regular.
As for GFX, card, if you want uber performance, by all means go for a 7800, but they have abundant amounts of overkill for most stuff out today. A 256MB 6800GT would be my weapon of choice :-)
Also, regarding the memory, most high end corsair sticks don't exceed PC3200. However, if I were you I'd avoid the gimmicky pro series and go for 2 sets of 1GB PC3200 TwinX-XL (ending up with 4 sticks of 512).
My reasoning behind this is mainly performance based. While you can get 2GB TwinX sets (2x1024), most of the latencies are a bit weak.
The PC3200 TwinX-XL is about as fast as it gets
Seeing as you seem to be building a whole system, I'd also recommend upgrading the naff on board sound with one of the soundblaster x-fi cards
Hope that helps some