Comments welcome. I've not upgraded for 3yrs so this is gonna be fun
I'm on a budget of ~£600 so although I know I can do better in some areas, I don't want to push the envelope 
Asus P5K Premium/WiFi-AP Intel P35 (Socket 775) PCI-Express DDR2 Motherboard
Intel Core 2 Quad Pro Q6600 "Energy Efficient SLACR 95W Edition" 2.40GHz (1066FSB) - OEM
2GB kit (1GBx2), Ballistix 240-pin DIMM, DDR2 PC2-8500 memory module (CL 5-5-5-15 1066Mhz)
or alternatively
OCZ 4GB (2x2GB) PC2-6400C5 Dual Channel Vista Gold Series DDR2 (OCZ2G8004GK) (CL 5-5-5-18 800Mhz DDR2)
GeForce 8800 GT 512MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail
Corsair HX 520W ATX2.2 Modular SLI Compliant PSU (CMPSU-520HXUK)
Arctic Cooling Freezer 7 Pro CPU Cooler (Socket LGA775)
Hitachi Deskstar 7K160 160GB SATA-II 8MB Cache - (OS & Apps)
Hitachi Deskstar T7K500 500GB SATA-II 16MB Cache - (Storage)
Cheers
Matt


Asus P5K Premium/WiFi-AP Intel P35 (Socket 775) PCI-Express DDR2 Motherboard
Intel Core 2 Quad Pro Q6600 "Energy Efficient SLACR 95W Edition" 2.40GHz (1066FSB) - OEM
2GB kit (1GBx2), Ballistix 240-pin DIMM, DDR2 PC2-8500 memory module (CL 5-5-5-15 1066Mhz)
or alternatively
OCZ 4GB (2x2GB) PC2-6400C5 Dual Channel Vista Gold Series DDR2 (OCZ2G8004GK) (CL 5-5-5-18 800Mhz DDR2)
GeForce 8800 GT 512MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail
Corsair HX 520W ATX2.2 Modular SLI Compliant PSU (CMPSU-520HXUK)
Arctic Cooling Freezer 7 Pro CPU Cooler (Socket LGA775)
Hitachi Deskstar 7K160 160GB SATA-II 8MB Cache - (OS & Apps)
Hitachi Deskstar T7K500 500GB SATA-II 16MB Cache - (Storage)
Cheers
Matt