What could be causing a slowdown across the board?

On a 1st generation Aluminium 24" 2.8GHz iMac running 10.5.2:

Menus open after a couple seconds or more. Switching apps results in
several seconds or even up to 30 seconds in some cases of the spinning
beachball. World of Warcraft is jumpy. Etc, etc.

I experienced this on the second of three 24" iMacs I had, but that one
was a serious lemon of course. This slowness on my third 24" iMac is new
and extremely annoying

I have enough HD space (little less that 100GB out of 500), CPU usage is
nowhere near maxed, memory usage is not maxed, drive activity is nothing
to write home about.

I've tried running all daily, weekly, and monthly housekeeping, I've run
Disk Utility (nothing wrong), fixed permissions (most minor of problems
found and fixed), cleared caches...

Could it be overheating? Fan speed right now is 1666rpm, CPU A is
currently at 49º celsius, GPU is 61º... hmmm... power supply is 81º - is
that normal?

Also - does anyone ever find on any Mac that stacks open immediately and
smoothly? Or the same with the dashboard? Or Exposé? The only places
I've seen these things work smoothly and responsively are Apple
keynotes.

TIA,
 Jamie Kahn Genet
-- 
If you're not part of the solution, you're part of the precipitate.