Obscene levels of volatility, an uncertain road to economic recovery, the nationalization of a couple of mortgage giants and slowing global growth should be more than enough reason to send even the most active and optimistic stock traders to the sidelines. But we know it won't.

One big reason is the safer alternatives for cash aren't all that compelling. We've searched high and low looking at savings accounts, money-market funds and munis, to name just a few, but let's face it: Inflation-beating yield is hard to come by when short-term interest rates are pegged at 2%.

True, those are snug and safe harbors that should protect principal in the turbulent waters of the current market. But they ain't gonna make you rich. With some hints that the economy and stocks could finally be poised for a comeback in 2009, we can understand how active traders must be itching to pull the trigger and shoot back into equities.

So let's remind those traders that in addition to diversification and keeping long horizons, it is oh so important to control costs. It's tough enough getting any return on all that capital you're putting at risk. Fees and commissions don't care; they're insatiable, eating away at your gains and fattening your losses.

SmartMoney Magazine's ranking of the top discount brokers takes into account a host of factors, including trading tools, research and customer service. For our purposes here we're assuming that you want to start moving some of that near-idle cash into stocks sooner rather than later, so we're just looking at the lowest possible commissions offered at the top-ranked discount brokers. They vary pretty widely and, as always, there are strings attached. (See chart below.)

Take TD Ameritrade (AMTD), for example. It ranked a respectable fourth out of 16 among discount brokers surveyed, but for our purposes here (lowest possible commissions among the top five overall), it was the most expensive. The catch? None, really. You're essentially paying a premium for the simplicity of its commission structure.

Compare that with E-Trade Financial (ETFC), which the magazine ranked first among discount brokers for a whole bunch of good reasons. And that $6.99 commission looks pretty attractive, too, until you realize you need to execute 1,500 trades a quarter to get it. Churn, baby, churn! That works out to more than 20 trades a day. Unless you are a professional trader, this is, um, inadvisable.

Then there are Fidelity and Charles Schwab (SCHW), which are both highly ranked and have much to recommend them. But you need to execute a lot of trades and/or house a hefty chunk of assets under their roofs to qualify for the cheapest fees.

That brings us to TradeKing, which came in third in the magazine's rankings but charges just $4.95 in commissions. Like most brokers, it tacks on an extra fee for trading in stocks priced below a certain level (in this case, a penny a share for stocks quoted at two bucks or less), but then do you really want to be speculating in Fannie Mae (FNM) or Freddie Mac (FRE) at a time like this?