Well if you can stretch your budget another £20, i would recommend this card:
The problem is that you've got yourself a PCI-Express mobo, and as that is the 'new' technology in terms of interface, then your going to find it hard to get a £50 PCI-Express card. ATI have only just really touched the PCI-Express technology with their X series of cards and in my opinion, nVida out perform the X600 and X700. So your really limited to nVidia.
As I said the card I stated is probably your best shot, but then I could be wrong. But for the price, I would go for that one.
But as you said your not going to be wanting to play the most up to date games, so you might get away with one of these:
But then again, it's up to you, and down to personal preference.
Hope that helps