With an expected vote due next Monday on the so-called Volcker rule, one question on big bank stocks is whether the impact of that rule has been fully priced into the stocks or investors are in for a big surprise. The new rule, the details of which are not yet available, is arguably the centerpiece of the Dodd-Frank financial reform legislation and is intended to help prevent future financial crises like the one brought on by the collapse of Lehman Brothers in 2008.

The rule itself is simple: Banks may no longer make investment bets that mix depositors’ funds with the banks’ own money. The devil will be in the details, and those have been devilishly difficult to address. Four federal agencies will vote to accept or reject a shaky agreement that has been pounded out over the past few years, and a fifth, the Securities and Exchange Commission (SEC), will enact its own rules at around the same time, according to SEC Chairman Mary Jo White.

Five of the country’s six largest banks have about $44 billion in annual revenue at stake in Monday’s vote. J.P. Morgan Chase & Co. (NYSE: JPM) generated about 12% of its total revenue in the 12 months to September 30 from it market-making (or principal trading) business. Bank of America Corp. (NYSE: BAC) and Citigroup Inc. (NYSE: C) both posted about 10% of their revenues from such trading. The big losers could be Goldman Sachs Group Inc. (NYSE: GS) and Morgan Stanley (NYSE: MS), both of which generated about 30% of revenues from principal trading.

Since the Volcker rule was first proposed for public comment in late 2011, shares in these five banks have risen by around 20% for Morgan Stanley and Citigroup, 33% at Bank of America and 45% at J.P. Morgan, and they remained flat at Goldman Sachs.

Wells Fargo & Co. (NYSE: WFC), the nation’s other big bank, is up nearly 60% in the same time period, but is not expected to be affected much because its trading activities are exempt from Volcker rule restrictions because Wells Fargo does not blend its own money with investors’ money on its private equity deals. The Volcker rule prohibits the blending and declares that because Wells Fargo uses 100% of its own cash to fund these deals, it is practicing merchant banking, which is specifically excluded from Volcker rule prohibitions against blending more than 3% of a bank’s money in private equity pools.

In our recent look at the best banking stock for 2014, we came down in favor of Citigroup on the strength of its turnaround and the stock’s potential upside of around 15%. Based on Wednesday’s closing price of around $52.04 and a consensus price target of $59.00, Citigroup’s implied gain is still 14%.

J.P. Morgan closed Wednesday at $57.19, and the consensus analyst price target of around $62.70 implies a potential upside of 10.4%. The bank has had more trouble in connection with its part in the Libor rate-setting scandal and is still the subject of more investigations both in the United States and Europe.

Bank of America, like Citigroup, faces the loss or at least the diminution of its trading revenues as a result of the Volcker rule. The stringency of the rule will dictate the impact on the share price, and how accurately investors have priced in that impact will determine the size of any potential gain. Bank of America shares closed at $15.63 Wednesday, up 0.58%, in a 52-week range of $9.95 to $15.98. Based on its consensus price target of around $15.30, the stock is fully valued.

Wells Fargo really has nothing much at stake in next Monday’s outcome on the Volcker rule voting. Shares closed up fractionally at $43.75, in a 52-week range of $32.69 to $44.79. Based on a consensus price target of around $46.00, the implied upside on Wells Fargo’s stock is about 5%.

Morgan Stanley stock closed up 0.52% Wednesday night, at $31.13 in a 52-week range of $16.69 to $31.85. With a consensus price target of $31.54, the shares are fully valued, and really have nowhere to go but down unless the Volcker rule is approved with much looser regulations than everyone expects.

Goldman Sachs, like Morgan Stanley, has depended on principal trading for about 30% of its revenues in the past 12 months, and both could be hit hard by the new rule. Goldman Sachs shares closed at $168.70, up 0.39%, in a 52-week range of $115.62 to $171.58. The consensus price target on the stock is $166.75, so Goldman’s stock, too, is fully valued.

Citigroup remains our choice among these big bank stocks, both because it has the largest potential upside and fairly low exposure to the impact of the Volcker rule. J.P. Morgan’s upside potential remains reasonably high, but its exposure to limits on trading and continuing regulatory issues make it a riskier. Wells Fargo, with only limited exposure to the Volcker rule and some remaining upside, may be a good pick for those who are risk-averse.

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