There is no jailbreak for 4.0.2.
The phone can do plenty without being jailbroken. I always jailbreak my devices, then get sick of lag and random apps crashing, and I always end up restoring.
If you have to ask what to do to get the most out of your phone(and not that there's anything wrong with that), then I don't think jailbreaking is going to really help you "get the most" regardless.
My advice: read the manual, google cool iphone stuff, and download top rated apps.