If you're running Tiger and don't mind reinstalling apps/moving files back and forth on an external, you can still perform a clean install with the $30 Snow Leopard disk.
If you do upgrade from Leopard or Snow Leopard, be sure to back up your files anyhow. There's no telling what can happen with an upgrade, even on a Mac (though I would recommend installing OS X through an upgrade 1000 times more than Windows). The Help Desk I work in at uni has seen more than a few upgrades go horribly wrong, and thankfully it's our policy not to perform heavy OS work unless the user backs up.