All depends on two things - what bus the motherboard has (PCI, PCI-E x1/x4/x8/x16, PCI Express? (I won’t mention VESA). Second is what is your budget. Once you know the facts of the first then the second will drive what you buy. Go to www.passmark.com and look at their graphics benchmarks - or www.tomshardware.com. Then go to Amazon and click, buy, boom.
Ah - one big thing - BIG thing - make sure you can get XP drivers for whatever card you buy. Check the company’s website - either going to be an AMD (used to be ATI) chip or NVidia - go to their sites for driver support.
Another gating factor is the size of the power supply and available connectors. Also, can the MOBO handle a bigger PS should it be needed?