An "older XP PC" suggests it will have PCI 2.0 slots, probably with PCIe or PCIx for video.
What's the PC spec? Make & model? Or mobo make, model and revision? From that you can work out what slots you have, and of what type. Then see what you are currently using to see what's left to plug a card into.
SIW (System Information for Windows) by Gabriel Topala is a freeware utility that will provide most or all of the above info, and is one of the few pieces of freeware I have felt grateful enough to donate real money to the author!
SD