The articles I've read say that your happiness is maxed out at an income of around $80-100k. This is enough to put a roof over your head, own a few nice things and be able to go on the odd holiday. Beyond that, all you get is nicer things, a nicer holiday and the ability to play keep up with the Joneses. Once you've had that Ferrari for a few weeks, it kinda becomes "just another car" (that you can't take shopping because the boot is too small and you don't want to park it where it can get damaged)
A lot of people on here have the goal of "work 1hr per day/week and be able to do whatever I want". Great goal, but from what I've read once you get there (and I haven't got there) it gets old quick. When I moved to London in 2001, the IT market was shit and I had some cash saved so I sat at home and played computer games for 6 months. It was great for the first few months, but quickly got boring. People that move to make a sea/tree change frequently move back within a few years because sitting around doing very little gets boring as fuck. So does continually traveling the world and living out of a suitcase for months at a time.
I think that even once I've "made it" I'll still be working at least 4 hours per day. Nothing too strenuous, but always have something on the boil - managing a team of people and client relationships, etc. Otherwise I'll get bored continually playing computer games, scuba diving, riding motorbikes, flying quad copters...
I've read the key to happiness is interpersonal relationships - caring for and having lots of people care about you