I'm a bit confused. Looking on dabs.co.uk they cost anything between 15
and 150 pounds (ok, the top end are those that output 700+, but even
exactly 500 can cost anything between 20 and 110 pounds).

Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).

About wattage - it only draws as much power from mains as needed
considering the components that have to be powered at that moment,
doesn't it?
How do I find out what wattage I need?

Tom