Before you buy, make sure you have a motherboard that has a PCI Express lane. You didn't list your computer model/specs, but I know most older computers have either plain PCI or AGP, which will severely limit your options.
That would typically be the case with a CSM (corporate-stable/consumer-stable) core - the *throwaway hardware* specification, in other words.
Even there, it's not always true - consider G3x and G4x, two Intel CSM chipset families. Quite a few motherboards with both chipsets have PCI Express x16 slots. (Mom's P4 has a G31 chipset, while my Q6600 has G41 - both have PCI-Express x16 slots that can be used.) But it is something that does indeed require user investigation.