Hi, would you need a hard drive, dvd drive, and case aswell?
Heres my suggestion ... glad to help
AMD Phenom II X4 955 Black Edition 3.2 GHz Socket AM3 8MB Cache
ASUS M4N72-E nForce 750a SLI Socket AM2+ 8 Channel Audio ATX Motherbaord
Corsair 4GB (2x2GB) DDR2 800MHz/PC2-6400 XMS2 Memory Kit CL5 1.8v
Arctic Power 700W PSU - With PCI-E, 4x SATA, 20+4, ATX12V, 8pin +12V Connectors
Western Digital WD5000AAKS 500GB SATAII 7200RPM 16MB Cache
Antec 300 Three Hundred Black Case
Pioneer DVR-S18LBK 22x DVD±RW DL & RAM Serial ATA Black Bare Drive
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Adds up to about £450, so you can still add more to reach your budget. Maybe a better gaming case or another 9800gt in Sli that would be awesome!