I know that some HDDs have SMART which helps give an idea over the overall health/stability of the drive, but are there other tools out there which might give a bit more of a comprehensive run down on the system and how its fairing?

I ask as Im trying to get it into my parents heads that their comp is so old that its not really viable to upgrade (uses SD ram etc), so if theres anything out there that I can use to show the age of the system in terms of performance, great.

Indeed, very often the cause of problems!

Otherwise the usual bumph, MemTest, Orthos, HDTach, etc etc.