Thanks, @drfrag. I didn't realise how driver dependent things like that were and thought it was mostly to do with the capabilities of the chip. I did look at NVidia cards but the ones available on eBay (I never use CEX they charge ridiculous prices at least here in the UK) all want
two PCI-e 6-pin connectors, my PSU only has one.
Replacing the PSU is a non-starter as cost is an issue for me and anyway, if the 6850 card worked I'd have been happy with it as it runs GZDoom OK until it overheats (I replaced the thermal grease; the problem is the fan won't turn on); so I'm really looking for like-for-like.
But now I know more about the subject, I can make better decisions. Thanks to all who replied. Now I will stop derailing this thread
