I just spoke to the people who supplied the hardware (this is a new system, only a few weeks old). Their tech support is excellent, although they don't have much Linux experience.
They say that while there used to be a problem with systems running more than 3Gb RAM, this is no longer the case. However, they are noting quite a number of problems where people are using Ubuntu 9.04 and believe that this is where the fault lies. The machine was fully tested with a Vista installation and the 8400GT card performed perfectly.
The suggestion was that I try loading the driver in Ubuntu 8.10, which they believe will solve the problem. I ran an 8.10 live disk session and installed the recommended driver (170), but this did not cure the fault, I still got low res graphics.
I have also come across references (http://tinyurl.com/6ztgse
) to an alternative driver, called Nouveau. This is an Open Source project and is allegedly available via the repositories, although it doesn't appear when I do a Synaptic search. Has anyone heard of, or used this?