Picture your 40th high school reunion. Within five minutes, somebody's passing around lake house photos, somebody's humble-bragging about early retirement, and somebody's quietly doing math on everyone else's 401(k).Here's the number everyone's curious about. Families headed by someone 55 to 64 with a 401(k)-style plan or IRA hold an average of $670,200 in those accounts. That's from the Federal Reserve's newest Survey of Consumer Finances, which came out Oct. 9.Before anyone spits out their punch, there's ...