What applications are you running? Some applications are known to "hog" resources until everything bogs down.
Top offenders I know of:
Safari - quit Safari periodically to free up RAM
Pages - if you leave a document open the amount of RAM Pages uses increases. Close Pages document periodically
AppleWorks 6 - The Recent Items aliases are never deleted, eventually bring AppleWorks & then the entire system to a standstill. Occasionally remove some or all of the aliases in Documents > AppleWorks User Data > Starting Points > Recent Items.