For a bit more you can add a 460 and/or better mobo, but you can keep going all day, try n stick to your original budget, the build above should be sufficient for most things still
HIS ATI Radeon HD 5770 1024MB GDDR5 PCI-Express Graphics Card £99.99
AMD Athlon II X4 Quad Core 640 3.00GHz (Socket AM3) - Retail £75.19
Patriot Viper 4GB (2x2GB) DDR3 PC3-12800C7 1600MHz Dual Channel Kit (PVS34G1600LLK) £63.44
Coolermaster Elite 430 Windowed Case - Black (with 500w Elite Power PSU) £62.99
Asus M4A78LT-M 760G (Socket AM3) DDR3 Motherboard £48.99
Samsung SpinPoint F3 500GB SATA-II 16MB Cache - OEM (HD502HJ) £31.98
LiteOn IHAS124 - DVD±RW (±R DL) / DVD-RAM drive - Serial ATA - 48x (CD) / 16x (DVD) 48x (CD) / 24x (DVD±R) /12x (DVD±R DL) 32x (CD) / 6x (DVD-RW) / 8x (DVD+RW) / 12x (DVD-RAM) - Internal - Black £11.97
Sub Total : £335.79
Shipping cost assumes delivery to UK Mainland with:
DPD Next Day Parcel
(This can be changed during checkout) Shipping : £10.50
VAT is being charged at 17.50% VAT : £60.60
Total : £406.89