That sounds like a dell. Updating is a good move as the integrated graphics are rather poor.
The potential downside being that if its the same one i'm thinking of, it has only PCI slots.
Either check on the dell site for the specs of your machine (i.e. what graphics ports it has) or pop the case open and have a look. If you're unsure which port is which, just jump on the net and look for pictures of PCI and AGP ports.
If it does only have PCI ports and you go to a store, make sure they don't give you a PCI-Express card. Happens too often
Depending on whether its for gaming use, you may want to wait and save for a new cpu+mobo. One that will support a better graphics output. High-street retailers seem to inflate the prices of high PCI cards due to a lack of demand. I've seen an FX5200 256MB card for around £70, yet the same store was doing a mobo+cpu bundle with far greater support for £100. Yes you still need to add graphics card to upgrade from the onboard, but it'll look/run a hell of a lot better. Also you dont want to spend your £70 on a passable card only to end up upgrading the rest soon after.