Tiger TV have an easy to follow walkthrough on installing a new graphics card into your PC:

Both Maplin and Insight are good places to buy a graphics card - Insight is a little cheaper than Maplin also (if you are prepared to wait for the card to be delivered!):

Of course, if you aren't confident, or just can't get it working - give us a call on 0845 310 2750 for some free advice or to arrange an appointment.