NEW YORK (CNN/Money) - A tsunami of cash is about to hit U.S. corporate coffers, thanks to a temporary tax break working its way through Congress.

But the jury's still out on how big that wave will be and how much it will actually impact the economy and corporate profits.

The House and Senate recently passed slightly different versions of a temporary tax break for U.S. multinationals, giving them a one-year window to reinvest overseas income in the United States at a super-low 5.25-percent tax rate, compared with the usual 35-percent rate.

In order to get the break, companies must present a plan for reinvesting that money in productive ways -- buying buildings, equipment or people -- in the United States. It's a plan lawmakers have been pushing for more than a year as a way to boost business spending and stop some of the outflow of U.S. jobs overseas.

The measure is part of a massive tax bill that still must be settled in a conference committee, and it may not become law until after Congress' August recess, at the earliest.

But it does seem likely to become law, so it's worth asking what its impact will be on corporate profits and the U.S. economy. The answer? Depends on whom you ask.

Economists at JP Morgan, based on surveys of corporate executives, believe the measure would result in an inflow of some $425 billion. Increased business activity would, in their view, boost the rate of gross domestic product (GDP) growth by a half percentage point in each of the two years after passage and create about 600,000 extra jobs.

Using JP Morgan's numbers, Allen Sinai, CEO of Decision Economics, recently estimated that the bill could boost the growth rate of gross domestic product (GDP) by 0.8 percentage points in 2005 and create 880,000 extra jobs in 2005 and 2006.

"The irony is that, now that the economy is much more certainly in a sustainable expansion phase, we can no longer say we really need this in order to jump-start investment spending in the economy," said Larry Horwitz, senior economist at Decision Economics. "But there's still a desire to unlock foreign earnings being kept overseas, and our view is that this bill will do that to a large extent."

It's worth noting that the Decision Economics study was sponsored by a group called the American Council for Capital Formation, a lobbying group for business interests. The measure was sponsored by Republicans and has been supported by several conservative groups.

"We will be deluged with capital during this period, and it will stay here," said Stephen Moore, president of the Club for Growth, a conservative lobbying group in favor of the provision. "This means the insourcing of tens of thousands of jobs for Americans -- it could be a significant thing in reversing the outflow of jobs and giving incentives for job-creating investments here in the United States."

In a classic illustration of the term "strange bedfellows," Moore shares common ground with Democratic presidential candidate, Massachusetts Senator John Kerry, who has also supported the measure.

But President Bush and the Treasury Department have broken with their conservative compadres and oppose it, saying it's unfair to companies which have paid taxes in the United States.

Meanwhile, other analysts aren't sure the impact will be quite as huge as its supporters suggest.

The bipartisan Joint Committee on Taxation (JCT), for example, has estimated that the measure would generate a much wimpier wave of cash, about $150 billion, while being a net revenue loser of some $3.5 billion in the next ten years.

The non-partisan Congressional Research Service (CRS) said in October that, no matter how much money flows into the United States as a result of the bill, it's unclear how much it will actually boost the economy.

The JCT, CRS and other economists suggest that, because money is fungible, there's simply no way of guaranteeing that repatriated earnings will be spent in exactly the way the bill's supporters hope.

"The earmarking requirement would have the effect of simply causing the company to declare that the repatriated funds are being used for a particular listed purpose, without necessarily altering the overall allocation of the company's funds between listed and unlisted purposes," the JCT wrote in a 2003 study.

Some analysts believe that, if businesses are just itching to spend money but feel constrained by the fact that they have to keep large amounts of cash overseas due to U.S. tax laws, they could simply borrow against their overseas cash to fund projects here.

In any event, multinationals haven't exactly been helpless victims of the cruel U.S. tax system. Way back in July 2000, in a study released by the National Bureau of Economic Research, Rutgers economist Rosanne Altshuler and Treasury Dept. economist Harry Grubert detailed several strategies companies were using to repatriate overseas earnings on the cheap.

"It's not plausible that capital spending has been weakened by the fact that multinationals are holding their funds abroad," said Jan Hatzius, senior economist at Goldman Sachs. "I just don't believe that story."

Hatzius also doubted repatriation would have much of an impact on corporate earnings, but not all analysts agree about that, either.

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Richard Bernstein, chief U.S. strategist at Merrill Lynch, released a note this week listing industries and specific companies that could see a profit boost from the measure.

Among industries, technology firms, industrials and consumer discretionary firms would seem to have the most to gain, due to an abundance of companies with a huge ratio of foreign income relative to total income.

But only six unprofitable companies would become profitable as a result of the measure, according to Bernstein's research: BMC Software (BMC: Research, Estimates), Delphi (DPH: Research, Estimates), MeadWestvaco (MWV: Research, Estimates), Navistar International (NAV: Research, Estimates), Novellus Systems (NVLS: Research, Estimates) and Schering-Plough (SGP: Research, Estimates).

And Bernstein warned that any such boost would also be temporary.

"The market might reward some companies during the second half of 2004 for producing better-than-expected earnings because of the tax windfall," Bernstein wrote, "but history suggests that the market will penalize those same companies if their earnings decelerate in 2005 from 2004's tax-induced growth."