I had the non-unique experience of being a multi-millionaire for 2 weeks in the summer of 1999. Which is to say that on paper, in terms of options and restricted stock, and stock which was currently owned but embargoed (due to my companies acquisition) was 'worth' millions.
I really had to sit back and think hard about what that meant, would I retire in 4 years?, keep working ? join a venture firm? The stock went from $120/share to $0.83/share before I could sell any of it so I never had to actually answer those questions but I found that how I felt when I was 'rich' was different than how I thought I would feel. I don't know if that is a common experience or not.
I find the idea that someone wouldn't really feel financially secure unless they had $1b in the bank sad. But I looked at what Google paid for Eric Schmidt's 'security detail' and I realized that at some point you become a 'soft target' for people who would acquire money through violence or extortion. I would hope to avoid becoming one of those targets. I've heard that if you are ever in danger of acquiring too much wealth you can 'fix' that by buying an airline. (with props to Sir Richard Branson)