Forgive my ignorance.

I have a 3GS with 3.1.3.

Is the upgrade to iOs 4.0 worth it? (Specifically, does it allow custom sounds by default...sms, mail, etc...or do I need to mod the current ones again?)

And if so, is there a jailbreak for it?

And if so again, will jailbroken apps that run with 3.1.3 run on iOs 4.0?