If you decide to build yourself, these parts perform pretty well and will leave some of your budget for Windows (if you need a copy) and peripherals/monitor. The Intel CyberPower uhgungawa linked to also looks like a pretty good deal.
CPU: AMD Phenom II 940 (3 GHz) [http://www.scan.co.uk/Products/AMD-Phenom-II-X4-940-Black-Edition-Deneb-S-AM2plus-30GHz-8MB-Total-Cache-HT-2000MHz-125W-Retail]
Motherboard: Gigabyte 790x (AM2+) [http://www.scan.co.uk/Products/Gigabyte-GA-MA790X-UD4-AMD-790X-AM2plus-PCI-E-20(x16)-DDR2-1066-1200-800-SATA-II-SATA-RAID-ATX]
Video Card: Radeon 4870 1GB (XFX) [http://www.scan.co.uk/Products/1GB-XFX-HD-4870-PCI-E-20-(x16)-3600MHz-GDDR5-GPU-750MHz-800-Cores-2x-DL-DVI-I-HDTV-HDCP]
RAM: 4GB Corsair DDR2-1066 [http://www.scan.co.uk/Products/4GB-(2x2GB)-Corsair-TwinX-XMS2-DDR2-PC2-6400-(800)-240-Pins-Non-ECC-Unbuffered-CAS-5-5-5-18-DHX]
Power Supply: Corsait 650W [http://www.scan.co.uk/Products/650W-TX-Corsair-PSU-single-12V-rail-energy-efficient-quiet-and-cool-fully-compatible-5yr-warranty]
Hard Drive: 640GB Western Digital Black [http://www.scan.co.uk/Products/640GB-Western-Digital-WD6401AALS-Caviar-Black-SATA-300-7200-rpm-32MB-Cache]
Optical: DVD-R (Samsung) [http://www.scan.co.uk/Products/Samsung-SH-S203D-BEBN-20x-DVDR-12x-DVDDL-DVD-plusRW-x8-RW-x6-SATA-Black-OEM]
Case: fan [http://www.scan.co.uk/Products/Antec-Three-Hundred-Black-Midi-Tower-Case-w-o-PSU]