Thanks for all the detailed advice people.

For info, it's an MSI Mega PC version 1. The
processor is 2.8 Ghz. Part of the problem is
that I'm using an nVidia FX5600 Ultra graphics
card which also has a fan on it. I don't really
want to replace the card. The other thing is that
I use a remote with the PC and the IR sensor is
on the front of the case, so I'm looking at a glass
door on the cupboard.

The discussion reminded me how loud my PS2
was before I put it in a glass-door-fronted cabinet
with the video and DVD. Now I can hardly hear
it with the door shut. I think I'll build the cupboard
with the option to add extra forced ventilation later,
and see how things go. I have a roll of unused
underlay in the loft, so that'll go in there as well.

Thanks again,
Al