You need to take into account whether you motherboard supports AGP or PCI Express.
Also some graphics cards require a power connector, so you need to check if there are any spare, and that your PSU is sufficiently powerful, at least 300W.
Then you have to decide whether to go for Nvidia or ATI based graphics cards.
Have a look at www.microdirect.co.uk under graphics cards for a some idea of cost, you should get a decent card for under �100