Wow, a whole lot of extra responses arrived a few days after I stopped looking at the thread! Thanks for the replies!
The motherboard is as shown here: http://www.gigabyte.com/products/produc ... id=3009#sp
I can't see any version number for the PCIe slot, so maybe that means it's version 1.0?. I'm not sure what the card needs, but I'll try to research it.
Since writing the original post, I discovered the PSU is a non-standard size, so I'm going to need to get a new case before I can replace the PSU. Fortunately, I have one I can get hold of in about a month - it will be a new experience trying to shift my motherboard into it! I think I'll proceed with buying a PSU - one that's good quality and has got enough grunt to put into my next PC when I upgrade.
Again, thanks very much for your comments!