You can pay anything for graphics cards - from about �20 to �400 !!! Im guessing that from what you said the PC doesnt have PCIexpress (or onboard graphics), so you are more than likely needing an AGP card. www.dabs.com seem to have a good range in mu opinion.
But bear in mind to check with your PC manufacturer what cards you can install, ie PCI, AGP or PCIe. You get what you pay for really. Its impossible to say what youll need without knowing the full spec of your system.