MY opinion: nVidia. I've found better driver user interface for nvidia. ATI's non-existence interface doesn't help if you get stuck.
Any Geforce 7 or 8 series card should be good. Go for a GT or GTX version. GS is entry level. I think you should get high-end 7 series in 40-50 pounds.
Check if your motherboard has PCIe support or not. Most cards need 16x PCIe. There are some models in AGP as well if you only have those. PCIe is better because it uses less power.
What is your power supply's maximum wattage?
Use this link
to calculate the required power for the card final system specs.