You can upgrade a mac, but it's risky. You need to make sure what you're buying will be compatible with OS X - ESPECIALLY video cards.
Now, take a look at my Hackintosh. I have Tiger 10.4.9 running on an HP dv8040us, an AMD system with 1GB of ram and two HDD's. THAT'S a Mac attack for you.

and yes, kernel panics do come from time to time, but they're actually pretty rare. No rarer than in Vista, at least, and Vista is installed on a machine made for it. The Darwin bootloader does act a little funky, though. I think that's more my fault than anything else.
As for the guy's comment that OS X is like Linux, I do agree to a point. The Terminal, however, is the only tru similarity - a lot of the commands are the same. The Terminal in OS X and Linux are probably derived from the same thing.
Thinking of trying to upgrade to Leopard, but it'll be tricky. Might just try and upgrade to 10.4.11.