OK here we go...
Powercolor ATI Radeon
HD 5870 PCS+ "CODMW2 Edition" 1024MB GDDR5 PCI-Express Graphics Card £299.99
AMD Phenom II X4 Quad Core 955 Black Edition "125W Edition" 3.20GHz (Socket AM3) - Retail £111.61
OcUK Cream 22" Widescreen LCD Monitor - Black £108.09
Microsoft Windows 7 Home Premium 64-Bit - OEM (GFC-00599) £82.99
Corsair Dominator 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit (CMD4GX3M2A1600C9) £79.89
OCZ StealthXStream 2 600W Power Supply £52.98
Gigabyte GA-MA78LMT-US2H 760G (Socket AM3) DDR3 Motherboard £51.99
Samsung SpinPoint F3 500GB SATA-II 16MB Cache - OEM (HD502HJ) £32.99
Xigmatek Asgard Midi Tower Case - Black £26.99
Logitech MX518 Gaming-Grade Optical Mouse (910-000616) £22.99
Saitek Cyborg V1 Gaming Keyboard £18.99
OcUK 22x DVD±RW SATA ReWriter (Black) - OEM £12.98
Sub Total : £902.49
Yup, that's right - a 5870! But note we still have a Phenom II 955 and 1600MHz ram. This will utterly p**s on anything else suggested so far. Nothing else is cheap rubbish either. I personally have that case and it's incredible for the money. OK he doesnt have his side window, but does he want to play games at their best or look at PCBs? It's £2.49 over budget, but you can probably find that down the sofa
That is an extreme example of what's possible with the spending geared towards gaming performance (and I would choose that spec in a heartbeat myself as for me performance is everything). However, you could always move down to 470 or 5850, which would still own 460/5770 and then have plenty spare to upgrade the other items, or add an aftermarket cooler for overclocking.
Keep thinking I must have forgotten something, but hopefully I haven't! The small savings where it doesn't really matter all add up, and this uses a lot of This Week Only special offers.
Liam