Don't even consider the 5200 as a games card it will struggle on any current game, ideally you want something in the fx5900xt range of cards. These will play all the latest games plus it should last you a good 18-24 months, or as byford says go for a 9800 card.
I'm looking at a recently released Nvidia 6800 (not the Ultra-too expensive). This is cheaper than the Radeon 9800 Pro or XT, but is considerably faster, plus it has all the latest technology. Apparently Nvidia is back on track with these latest series.
I am not sure of the price, but it would be coming with the new power station package...a Holly computer. This is the only (well-known) company I can find that is using them. They have an amazing range of potential configurations for their systems.
On their gaming system (Intel P4 Silver/Ivory) click here configuration, the 128 Mb Chaintech Geforce 6800 is £65.86 less than the 256 MB Radeaon 9800Pro , and the 128 Radeon 9800 pro is £81 less. And the Radeon 9800 Pro AiW is £11 more thean the 256 MB version.
If the Nvidia card is really that much better than the 9800 pro, then £16 extra is not much, I reckon, given that you will have the latest graphics technology, while the Radeon will soon be obsolete.