Yeah, there is definitely a limit to how much you can gain which is useful to spend purely on your own confort and entertainment. When you think of worthy projects you could contribute funds to though I don't think there is any limit to the amount of money that would be useful.
I posted this as its own link, but it's worth reposting here: Jaan Tallinn, founder of Skype, at the most recent Singularity Summit, talks about what to do with your money and how to make a difference.
Not a billion dollars.
But yeah I always wondered what the threshold of wealth is when you literally can do anything you want without any thought to how much.