Have you got enough RAM? It still matters, but even then I've never known a Mac to crash. Maybe an application gets confused because you tell it to do something and don't give it a chance to do it before you tell it to do the next thing. Safari once in a while for me becomes unresponsive and I had my TiBook get very cranky this week because I added a new FireWire external drive and it was still formatted for PC.

Generally when my Mac slows down under X.4.9 I just quit all applications and restart and that restores things to normal.

There are also things like unresponsive web servers that make one think the Mac has the problem, but normally Safari or Mail will ping a site for at least a minute before you get the error message about the site's server being unresponsive.

Sending large files to some desktop printers is not a particularly speedy process.

I do have an impatient friend who, when something doesn't happen immediately, clicks on menus and buttons and the desktop, sort of like drumming her fingers on the desk, and then the poor Mac tries to remember all those little commands and that really slows things down.

Have you talked with people about what you were doing when the Mac appeared to crash?