Billions of dollars are at stake as boomers decide what to do with the $5.3 trillion they've invested in company-sponsored 401(k) plans when they retire. Leave the money where it is? Roll it over to an Individual Retirement Account (IRA) at a financial firm? For many, it's a head-scratcher.Merrill Lynch financial advisor Marie Vanerian, who is based in Troy, says there are a couple, key things you should do before deciding what to do with a 401(k).