Asus do these: 
http://www.overclockers.co.uk/showproduct.php?prodid=FS-062-AS&groupid=701&catid=110&subcat= in a P45 chipset (which is slightly better than G31) - but that, coupled with a dual core 5x00/8400/7400, and 4Gb of RAM and the rest of the gubbins will be perfect.
	Intel Core 2 Duo E8400 LGA775 'Wolfdale' 3.00GHz (1333FSB) - Retail	 	£126.99
(£110.43) 	£126.99
(£110.43)
XFX ATI Radeon HD 4870 1024MB GDDR5 PCI-Express Graphics Card	XFX ATI Radeon HD 4870 1024MB GDDR5 PCI-Express Graphics Card		£99.98
(£86.94) 	£99.98
(£86.94)
Western Digital Caviar Black 640GB SATA-II 32MB Cache - OEM (WD6401AALS)	Western Digital Caviar Black 640GB SATA-II 32MB Cache - OEM (WD6401AALS)		£52.99
(£46.08) 	£105.98
(£92.16)
Intel Pentium Dual-Core E5200 "LGA775 Core 2" 2.50GHz (800FSB) - Retail	Intel Pentium Dual-Core E5200 "LGA775 Core 2" 2.50GHz (800FSB) - Retail		£51.99
(£45.21) 	£51.99
(£45.21)
Patriot Viper 4GB (2x2GB) DDR2 PC2-6400C4 Low Latancy Dual Channel (PVS24G6400LLK)	Patriot Viper 4GB (2x2GB) DDR2 PC2-6400C4 Low Latancy Dual Channel (PVS24G6400LLK)		£42.99
(£37.38) 	£42.99
(£37.38)
LG GH22NS40 22x DVD±RW SATA Dual Layer ReWriter (Black) - OEM	LG GH22NS40 22x DVD±RW SATA Dual Layer ReWriter (Black) - OEM		£18.99
(£16.51) 	£18.99
(£16.51)
  		Sub Total : 	£388.63
Shipping cost assumes delivery to UK Mainland with:
DPD Next Day Parcel
(This can be changed during checkout) 	Shipping : 	£9.50
VAT is being charged at 15% 	VAT : 	£59.72
  	Total : 	£457.85
CPU is a tad overkill, could probably even drop that to a E5300 and it'll run perfectly for what you need. Cut out and switch some drives if you want. But that's what I'd get.