Have a look at www.overclockers.co.uk
for the deals they have on (Leadtek sale is on now for the GPU's.) Generally OCUK are quite expensive, but they have fast bandwidth so are ideal for specing out your PC.
Pick out a nice ATX case that you like, if you need more harddrive space there is room in the budget for that. The CPU's are on a deal this week as well. If you wanted the 4000 it is £76 more. The RAM is also overclocking RAM, if you never ever plan on overclocking there are cheaper alternatives.
AMD Athlon 64 3700+ San Diego 90nm (Socket 939) - Retail (ADA3700BNBOX) (CP-121-AM)
DFI LanParty UT NF4 Ultra-D (Socket 939) PCI-Express Motherboard (MB-014-DF)
Leadtek WinFast GeForce 6800 GT 256MB DDR3 TV-Out/Dual DVI (PCI-Express) - Retail (GX-049-LT)
OCZ 1GB (2 x 512MB) PC3200 Dual Channel Platinum Series EL-DDR REVISION 2 CAS2 (MY-048-OC)
OCZ ModStream 520w ATX2.2 Power Supply (CA-017-OC)
Seagate Barracuda 7200.7 Plus NCQ 80GB ST380817AS SATA 8MB Cache - OEM (HD-003-SE)
NEC ND3540 16x16 DVD±RW Dual Layer ReWriter (Black) - OEM (CD-019-NE)