It depends on what slots are available on your motherboard (PCI, AGP or PCIE), the power of your PSU, and your price limit.
Post the Vista's full system specs. You can get these details using
Everest (look under Computer > Summary).
Open the case and post back with the details from the PSU label (make/model, total watts and amps for +12V). This will tell us if the computer can handle an upgrade.