My guess is it almost certainly an AGP card as your PC probably has an AGP slot.
Here is a basic AGP card for around �27 (you can probably get one for less if you hunt around).
http://www.ebuyer.com/product/149296
When you come to replace the graphics card FIRST uninstall the drivers for the old card and then set Windowsr to basic VGA settings.
Then power off PC, take out old card, and put in new card.
Hopefully Windows will recognize it and display a basic VGA window.
(The problem you can get at this point is that Windows does not recognize the card so nothing is displayed on the screen so you cant actually DO anything).
Assuming you have Windows displayed then you need to install the driver for the NEW graphics card, which should allow you to set a decent resolution.
I would not spend too much on a graphics card because the rest of you computer will not be up to it.
Also some better graphics card require a special power to go to direct to your card, and your older power supply will probably not have the connector.
Buy the cheapest graphics card you can, then save up for a new PC.