That is some of the best advice.
While a i7 system is very tempting, I'd say stick with a high end 775 system i7 is just too expensive atm.
LN16718 Antec 900 - Nine Hundred Ultimate Gamer Case with 200m Top Fan w/o PSU £71.19
LN19302 4GB Corsair (2x2GB) TwinX XMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-18 (two lots for 8gb) £75.07
LN22387 640 GB Western Digital WD6400AAKS Caviar Blue, SATA 300, 7200 rpm, 16MB Cache, 8.9 ms £49.32 (I think this is the good single platter one, I'm sure I'll be corrected if it isn't
)
LN23007 Samsung SH-S223F/BEBE 22x DVD±R, 12x DVD±R, DVD+RW x8/-RW x6, SATA, Black, OEM £14.81
LN24601 DFI Lanparty DK P45-T2RS Plus, iP45, S 775, PCI-E (x16), DDR2 1066/1200, SATA II, ATX £100.22
LN25322 1GB Asus HD4870, PCI-E 2.0(x16), 3600MHz GDDR5, GPU 750MHz, 800 Cores, HDTV/ 2x DL DVI-I, HDCP £198.89
LN25428 ZEROtherm ZEN FZ-120 Quiet Tower CPU Cooler Intel LGA775 £24.14
LN21951 525W Enermax MODU82+ EMD525AWT Modular Triple PCI-E Rail, ATX12v 2.3 , 88% Eff, £73.01
Total £527.53 then add
LN19488 Intel Core 2 Quad Q6600, Kentsfield Core, S775, 2.40 GHz 1066MHz 8MB Cache, 9x Multiplier, Retail £147.07
or
LN24841 Intel Core2 Duo E7400, Wolfdale Core, S775, 2.8GHz, FSB 1066MHz, 3MB Cache, 10.5x Multiplier, Retail £112.00
And
LN17166 Microsoft Windows Vista Home Premium SP1* 64Bit 1Pk OEM (DVD) £87.27
Total is under £865 with the quad or £835 with the c2d
So that leaves extra left over for
more Hard drive space (ether more hd's in a raid or a larger hd)
Sound card, if onboard isn't enough
Speakers, you'll be hard pressed to notice the sound difference between onboard and seperate sound card on cheap/weak speakers
mouse + keykeyboard + any other little things