Well despite the helpful comments
I went ahead with it and got a 160GB drive from Novatech. The only snag is the old BIOS doesn't recognise the full drive capacity so I only get 128GB available but that is 3x what was in there and plenty to see this old thing out for its intended use. I think there are possibly work-arounds to upgrade the BIOS but I can't be bothered.
A worthwhile upgrade and after a full reinstall with Win XP SP3, AVG and most of my main programs, there is still 95GB free and it runs at least twice as fast as it did before, doesn't crash every five minutes and starts up and shuts down about 5x as fast as it did a week ago, so a "descrambling exercise" is worth the effort and my E-TEC diagnostic unit is much better than it used to be