If you are wanting to do 3D work / gaming then you should go for a high end card like the nVidia 280 or the Ati 4870
If you just want to do 2D work (web / print) then a mid range card will be fine like the nVidia 9800 or the Ati 3870
With new graphics cards these days you also have to make sure that:
A) they will fit in your case as newer cards are bloomin' big!
B) that you have a big enough PSU (power supply unit) that has a specific power connector that connects to your card as most modern cards require power direct from the PSU and not just through the motherboard. You really want a minimum rated PSU of 600W.
C) make sure your motherboard has the right type of slot for you card. Most new cards require PCI-e connection.
**Don't go for a cheap £20 PSU becuase they have a tendancy to blow up and will take the rest of your PC components with it. Generally you want to spend at least £50.