Have you tried using a system utility like Onyx?
It repairs permissions, and all that other fun stuff. There is actually a video about what it does in more detail on mactuts.
I'm also fairly certain that they have different downloads for the different versions of Mac OS so you should be able to find something that can help you speed up your computer. Programs like Onyx can work to do most (if not all) of the software fixes that pietrodn suggested.
I also disabled the dashboard using the terminal type the following into the terminal:
defaults write com.apple.dashboard mcx-disabled -boolean true
and then type:
Also one thing that helped me speed up my MacBook Pro was moving my iTunes library to an external harddrive. Just a thought but it might help.