JB007,
Dont buy off the shelf !! You will be wasting your money.
Go to
www.scan.co.uk and click on their Today Only link and check out the deals.
Make sure the card is at least a GE Force II card with minimum of 32mb ( get 64 if you can afford it )
£160 should do it for you.
Also
www.dabs.com are good.
Memory is VERY cheap...... get as much as possible but make sure it is compatable with your exisiting system before you buy !!