I have the 4850 and it's the best card for your money. I have mines coupled with an e8400 and can max out any game bar crysis (Have to turn down a few settings) so with a 5200 you should manage to play all the latest games (make sure you overclock it though

) If you could stretch a bit more I'd go for a better PSU and case, the 430 earth watts is a good PSU but once you start overclocking the CPU and/or graphics card then you might want a bit more power. I've got a 500w supply and it manages to power my system quite easily. So maybe go for a corsair PSU and Antec 300 case.
Your basket
Product Name
Antec Three Hundred Ultimate Gaming Case
(£46.99) £39.99
Intel Pentium Dual-Core E5200 "LGA775 Core 2" 2.50GHz (800FSB) - Retail
(£58.74) £49.99
Asus P5Q Intel P45 (Socket 775) PCI-Express DDR2 Motherboard Asus P5Q
(£88.11) £74.99
G.Skill 4GB DDR2 PQ PC2-8000C5 (2x2GB) CAS5 Dual Channel Kit (F2-8000CL5D-4GBPQ)
(£58.74) £49.99
HIS ATI Radeon HD 4850 512MB GDDR3 TV-Out/Dual DVI/HDMI (PCI-Express) - Retail
(£112.79) £95.99
Seagate Barracuda 7200.11 500GB SATA-II 32MB Cache - OEM (ST3500320AS)
(£44.64) £37.99
Pioneer DVR-216DBK 20x DVD±RW SATA Dual Layer ReWriter (Black) - OEM
(£15.85) £13.49
Arctic Cooling Freezer 7 Pro CPU Cooler (Socket LGA775)
(£17.61) £14.99
Corsair TX 650W ATX2.2 SLi Compliant PSU
(£64.61) £54.99
Sub Total : £432.41
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £12.95
VAT is being charged at 17.5% VAT : £77.94
Total : £523.30
It's a bit over your budget.