System Maintenance and Optimization Tools

Alright,
My system has been acting funky and seems to have slowed down a lot, even after a re-install. I'm guessing it is just age and the constant running has deteriorated its speed a bit.

What are some apps/tools I can use to analyze my system to see where there may be issues as well as optimize it?

Specs:
1.25G4
1.25GB RAM
80GB HD (20gb available)
OSX 10.4.2

I'm thinking of buying a faster HD or two and some new memory (even though Id prefer to use my surplus, see other thread). But that needs to wait a bit.

Advice please.

Thanks