Simple if you Motherboard is AGP Or PCI then you have a choice from Hundreds of cards. If you PC is AGP then a maximum size of 512MB is allowed. If your PC is (PCI-E16x) Or the new (2.0 PCI-E16x) Then you have a choice of cards from 32MB to 4GB. Best thing to do to find out what card you have, Is go Control Panel, Then look under your display setting look for Graphics settings and you should find details on what your card is, Who made it speeds & amounts. Go from there buy a card to Match.

PS you can download a Free Program called Everest it tells you everything & anything about your computer that you could even need to know.

Look at your motherboard, it should be AGP or PCI-Express and both slots are rather different,
when buying a card just pay attention to 2 things:

-Motherboard(it should be the same type, PCI-E or AGP)
-Power Supply(500+ watts minimum if it’s gonna be a mid to high range card)

For the Power Supply, just look at the sticker at the side of your power supply(you need to open the computer) and see how much watts does it supply.
For the motherboard, look at the manual, model of the computer and search it on the internet,
or just look at the slots, if it has a large slot(like the one on the PCI) followed by a little slot, then it’s PCI but I recommend looking at pictures, it’s hard to describe.