I don't know all of the specs on the computer that I've used Apple OS X 10.6 Snow Leopard on, but it is an Apple G5 desk top with a good amount of juice to it. I really don't think that it will make too much of a difference in terms of performance if you're upgrading from 10.5. As far as upgrading from older versions than that, I would make sure that your system has enough processing power to handle 10.6 Snow Leopard before making up upgrades, as it does require a superior system to run it on.
You can upgrade to Apple OS X 10.6 Snow Leopard from 10.5 Leopard for very cheap, I would definitely consider doing it. However, make sure that whatever software you're running will be fully supported and compatible before making the upgrade, as I haven't made the leap myself yet for this reason. Digidesign won't yet support use of Pro Tools 8 on Snow Leopard as far as I know, so this was both my reasoning for not yet upgrading to Snow Leopard. There are a lot of new features with Snow Leopard and it runs smoother, but it isn't necessary to have. Having said this, if you won't experience compatibility issues, I'd recommend taking the plunge and upgrading as this is definitely their best operating system to date.