Thanks repilce, maybe trying an older driver is an obvious solution to some, but I hadn't thought of it - I just sat there working through the readme FAQs for the latest driver to no avail.
To surmise in case it helps anyone:
- updating the driver and changing "nv" to "nvidia" in the config file prevented the X Window system from starting
- the FAQ said that the installer should create some files in the "dev" directory, it didn't. It also said that older copies of the libraries could make it go wrong, and that you should rename them XXXwhatever, I tried that but it still didn't successfully put any files in the dev directory.
- changing the config file back to "nv" got the X Window system working again, but at the expense of rubbish performance again
- downloading an older version of the driver made it work all fine and dandy
I am using Mandrake 9.2 (kernel version 2.4) with a GeForce 4.