Try to run the free Belarc Advisor click here or Everest click here on the PC & let us know the motherboard make & model. We can then advise on which cards will be compatible, plus even provide you with purchase links.
eBay is a good place to look - you're likely to find several older models at bargain prices. Someone here had a problem with his graphics card a few weeks ago. He searched eBay & got quite a good one (though low spec compared to today's standards) for £6 delivered. G