Tony, true - unless it is a Compaq or Dell, which are usually impossible to upgrade as you suggest, due to proprietary components and fittings.
I am also loath to part with older kit, and I do tend to run kit of a similar spec. to the above well past its use-by date. But then I accept that a 600MHz cpu, 768MB RAM and AGP2 graphics card with 64MB RAM is fine for PPRuNe, email, WP, etc. but never going to run games and video, which is fine because I don't want to most of the time.
One of my old PCs is running happily as Win2K server, and another is ideal for one of the boys' bedrooms for homework - precisely because it won't run youtube and games. Only problem is they keep coming downstairs to use one of the faster PCs!
SD