What to do when your mac slows to a crawl?

I.e., what to do when your Mac slows to a crawl. One thing I usually do before logging out, rebooting, defragging, reformatting my hard drive and reinstalling, etc., is going to the Activity Monitor in the Utilities folder (running "top" from the command line works too), and seeing if some process is slurping up all your CPU cycles. Sometimes an application (Word v. X and Filemaker 5.x are both likely culprits) will suck up all the available processor cycles and bring your system to its knees. Oftentimes quitting a greedy application will speed things right back up.
If your Mac is slowing to a crawl, then you a) have too many programs open and need to buy more RAM, or b) created and installed your own spyware ;)
Defrag issue:- Isn't setting permisions good enough? Like it'll set a file type correct etc.... ?

And now another:- Is it best to use Secure Erase rather then plain Empty Trash? Wouldn't it be the same as using Erase Free Space you can find on the startup DVD and other Disk Utilities Applications. I think with that clear space it can better a performance yeah?
I did a hardware test yeaterday with the CD I got with my iBook (something so obvious i overlooked it), ended up showing that there was an error with the hard drive, I called apple and they said this particular error means the hard drive will need replacing, iBook is still under warranty, hopefully problem solved.

thanks for all your input

i find that programs of an ilk don't running together - photoshop, and illustrator are fine - designed to work together, both open at the same time, and both do different things. however! if iPhoto is open at the same time as photoshop (both rendering bitmapped images, lot of RAM needed), both are sluggish. to speed up photoshop, i close iPhoto.

the activity montitor is very useful - keep it on processor history viewable in the dock to keep track on what's really happening. it's very good to check rogue programs