For banking customers, the final act of the Great Deregulation Drama is ready to unfold. The government is ending all formal restraints on small accounts.

Starting Jan. 1, even the smallest depositor (with less than $1,000) can -- theoretically -- have a money-market deposit account, a seven- to 31-day certificate of deposit and a market-rate, interest-paying checking account (Super NOW). Starting March 31, all interest-rate ceilings will pass into history. Banks and S&Ls will be able to pay what they want, even on small passbook accounts.

The $64 question, of course, is: What do the bankers want to pay? The answer appears to be: Not much, if you have only a small amount of money.

It's entirely possible that this final phase of deregulation will raise the fees charged by banks and S&Ls faster than they raise interest. You'll have to play the system like a piano to come out ahead.

Here's what's going to be happening early this year:

Your ready savings: Interest rates on money-market accounts now average 6.6 percent, according to the Bank Rate Monitor, compared with 5.25 to 5.5 percent on passbook accounts. Yet $301.6 billion is still carelessly stashed in the lower-rate deposits.

Many banks and S&Ls like that just fine, and won't be advertising higher-rate savings to their passbook depositors. To discourage switching, some institutions will retain their present, $1,000 minimum deposit on money-market accounts. Some will lower the minimum to $500 or $250, but will pay no more than 5.5 percent interest on the first $1,000 in the account. Some may pay no interest at all on deposits under $100 or so.

You also can expect to find higher fees for money-market deposits under $500 to $1,000, and higher charges for writing checks against your savings, or making automatic teller machine (ATM) withdrawals.

What to do?

Move your money out of passbook accounts, if you have more than $1,000 in savings. The higher interest rate probably will offset any fees.

But keep an eye on the "tiers" in money-market savings. Increasingly, banks are paying low rates on, say, the first $1,000 in your account; a market rate up to $50,000 or so; and above-market rates on higher amounts. But they calculate your interest payments in various ways.

Some will pay you the lowest interest allowed if your monthly balance falls below $1,000 for just one day -- which can greatly reduce your total earnings. Others will base your interest on the daily balance or average balance in your account for the month.

Some, in the example above, will pay a high rate on your whole deposit if it exceeds $1,000. Others cut your earnings by paying high rates only on the portion above $1,000. If you have less than $1,000, compare passbook and money-market savings accounts carefully. Very small depositors may find that they net more, after fees, by sticking to passbooks.

Both large and small savers earn more by switching their money out of banks and into money-market mutual funds, whose current yield, according to the Donoghue Money Fund Report, is 7.2 percent.

Starting March 31, some institutions may be offering 6 percent or so on passbook accounts, says Edward Katz, president of The Amalgamated Bank of New York. You also may find some extras thrown in to discourage you from seeking higher-interest deposits: perhaps no-fee credit cards or free travelers checks.

If you have less than $500 to $100, however, passbook accounts may be a dud. Many institutions pay no interest at all on such small amounts, and some even charge fees. If you find that your small passbook account is costing you money, look for another bank or S&L; consider a credit union; or -- last resort -- stash the money in a coffee can on the closet shelf.

*Your checking account: Banks gradually will combine Super NOW and NOW accounts into a single, interest-paying checking account. But you may get no interest at all on balances under $1,000, and extra fees may be imposed if you write your account down to $100 or less. Fees also are going up for writing checks and using ATMs.

If you keep a substantial amount of money in a checking account, the new interest-paying NOW is the place to be. But if you pretty much empty your account by the end of the month, you may find yourself better off with old-fashioned, no-interest checking. It does you no favors, but may cost you less.