If all you need is a Mobo, CPU, RAM and a Graphics Card I would suggest the following.. I would think this would be able to handle what you asked for..
Intel Core 2 Duo E6600 "LGA775 Conroe" 2.40GHz (1066FSB) - Retail £127.99 (£150.39)
Gigabyte GA_P35C_DS3R (Socket 775) PCI-Express DDR2 & DDR3 Motherboard £81.99 (£96.34)
EVGA GeForce 8800 GTS 640MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail (640-P2-E821-AR) £219.99 (£258.49)
Crucial Ballistix 2GB (2x1GB) DDR2 PC2-5300C3 667MHz Dual Channel Kit (BL2KIT12864AA663) £59.99 (£70.49)
Sub Total : £489.96
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £8.95
VAT is being charged at 17.5% VAT : £87.31
Total : £586.22
The above spec is presuming that you already have Case, HDD, etc etc.... [Your post is not 100% clear if you wanted a whole new system so only suggested what you asked for in the title.] There is not much point going for the E6700 since the difference is minimal in performance but rather large in cost. Also, everyone is saying that with just a tad bit of OverClocking on the E6600 and you will be doing the E6700 speed. [Not done any OC myself so can't vouch for it!]
The Mobo is future proof in such regards that it will support the upcoming 45nm platform when it comes available and it supports both DDR2 and DDR3. The RAM's I have been told is great for OC if you are into that [and so is the Mobo]. Only picked 2GB since you are still on XP, when you decide to upgrade to Vista it will be easy to just add another 2GB [but only if you upgrade to the 64 bit version, with 32 bit 2GB should still be fine]. I would have picked the LeadTeak GTS since thats £30 cheaper, but its out of stock and not sure if you are about to order this now or not so picked whats available. If you would wait until it comes in you could go for the LeadTeak GTS and then add a
Noctua NH-U12F Heatsink if you are going to overclock. The total would then be £595.61 including Delivery.
Then again if you are after a whole new system the above spec is going out through the window in record speed!
If not.. I hope the above spec helps you along the way.. [but please wait for the more wise and experience users to give you their opinions also!

]
Cheers
Tom