Invest in what you know. It's an old truism, but the disarray on Wall Street gives it new meaning.

With Uncle Sam seizing control of American International Group (AIG) and three of the nation's five major investment banks gone, even the stalwart Goldman Sachs (GS) is looking more suspect, as is its lone surviving competitor Morgan Stanley (MS) . It's possible the feds (and thus taxpayers) will have to clean up Wall Street's mess by buying up more bad assets to stop the bleeding.

All difficult to believe, yes, but it goes to show just how cursory our understanding is and was of all the crazy financial instruments spawned during the housing bubble. AIG didn't fail because of its plain-vanilla consumer insurance business, but rather because of the complex credit default swaps it traded in that were tied to mortgage debt.

While hard to fathom at the moment the debris has to settle eventually, and when it does banks that "regular" people use will emerge thumping their chests. Gone for some time are the glory days of banks that "sophisticated" investors used to hedge, insure and securitize.

Instead, it'll be back to basics with Main Street-oriented shops like Bank of America (BAC), J.P. Morgan Chase (JPM) and Wells Fargo (WFC) . Their core business is getting as many deposits as possible, which means big balance sheets. Chase's bank deposits total $722 billion, Bank of America has $786 billion, and Wells Fargo, $339 billion.

So while Bank of America is buying Merrill Lynch (MER) and Chase has Bear Stearns, their core is still deposits. As Bank of America Chief Executive Ken Lewis happily reminded the world this week, "For seven years now as CEO, I have said commercial banks would eventually own investment banks because of the funding issue."

For now, we'd focus on these big commercial banks. Barclays (BCS), which is buying bankrupt Lehman Brothers (LEH) U.S. broker-dealer business, is a bit riskier. Some analysts have their own credit-related qualms about the firm.

But you don't need to stick your neck out amid this chaos and buy the individual stocks of Bank of America, Chase or Wells Fargo in order to ride their wave. A handful of good and cheap mutual funds can give you ample exposure, including Vanguard Total Stock Market Index (VTSMX) and T. Rowe Price Equity Income (PRFDX) . Franklin Income A (FKINX) also makes the cut, but bear in mind it recently listed Lehman and AIG among its top 25 holdings and charges a load. For details, see the chart below.