Can someone explain to me how significant the recent economic turmoil is?

Fannie Mae and Freddy Mac have been taken over by the government, Lehman Brothers has filed for bankrupcy, and Merrill Lynch has been sold to Bank of America. I'm not a customer of any of those companies, but does this affect me indirectly? Are stocks and investments involved with Lehman Brothers now effectively void? If not, how long would a firesale last? Exactly how do investment banks like Lehman Brothers and Merrill Lynch differ from Bank of America, Citigroup, Chase, etc.?