How much federal tax will you be paying over the next year or two . . . or five? It's really hard to tell these days, but what happens on Capitol Hill in the next two weeks will supply key pieces of the picture.

There are two big tax-cut bills pending in Congress, one already passed by the Senate, the other approved by the House Ways and Means Committee and awaiting floor action when that chamber returns this week.

Both bills include tens of billions of dollars in tax cuts, but they differ widely in how those cuts are to be handed out. These differences will have to be hammered out before a final bill can be passed, and that process is shaping up as difficult and contentious.

Lawmakers could, of course, wait until next year, but that would mean allowing a number of tax breaks to expire Dec. 31 and then possibly restoring them retroactively. This happens often enough, but it drives taxpayers and their advisers -- not to mention the Internal Revenue Service -- nuts in trying to plan.

It would also push this divisive issue into an election year, which many members would like to avoid.

So here's what we're looking at.

Both measures would extend a number of cuts that were enacted temporarily in a series of bills dating back to 2001.

These include allowing small businesses to "expense" (deduct immediately) up to $100,000 in equipment purchases, giving low-income taxpayers a "saver's credit" of up to $1,000 for contributions to retirement savings accounts, and extending an "above-the-line" (i.e., available to those who don't itemize) deduction for college tuition.

Both bills would also extend through next year the right of taxpayers to choose to deduct state and local sales taxes instead of income taxes, if they find that more advantageous. And they would extend through 2006 the $250 deduction for teachers' classroom expenses, along with the credit for first-time home buyers in the District.

But while the House bill would also extend for two years the maximum 15 percent tax rates on capital gains and stock dividends, which are now scheduled to expire after 2008, the Senate measure would not.

On the other hand, the Senate approved a one-year extension and slight improvement of the "patch" in current law that eases the impact of the alternative minimum tax, but the House bill doesn't include it.

The problem is that adopting both those provisions would jack up the cost enough to scare a lot of people, even Republicans, and also it would blow right past the Senate's budget limit, possibly derailing the bill.

The AMT patch extension would cost the government more than $30 billion, and the low capital-gains and dividend rates are a $20 billion revenue hit over five years. The timing is different, but the measures already add more than $56 billion to the deficit over five years.

Compounding the chambers' problems, the Senate measure is much broader than the House's. It includes further tax relief for Katrina/Rita/Wilma hurricane victims and allows an itemized deduction for mortgage insurance on a taxpayer's residence.

The Senate bill also includes more than a dozen items relating to charitable giving, some of which would tighten rules while others would ease them.

On the loosening side, the bill would allow taxpayers who do not itemize a limited above-the-line deduction for charitable gifts, and it would enable older taxpayers to make tax-free gifts directly from their IRAs to charity, rather than withdrawing the money as a taxable distribution from the IRA, then donating and deducting it.

On the other side, the bill would crack down on charitable donations of facade easements on buildings in historic districts, limit deductions for donations of clothing and household items, and require written substantiation -- either a specific receipt or a canceled check -- for all cash donations.

The Senate measure also includes provisions that raise money for the government. Many are technical and apply to businesses, but a couple could bite a lot of taxpayers if they make it into law.

One expands the "kiddie tax," which now requires that "unearned" income such as interest and dividends of children under age 14 that exceeds $1,600 be taxed at the parents' rate. The bill would extend that treatment to children under 18, and would raise $2.1 billion over 10 years.

The other, a cynical one-shot deal that would raise $5 billion next year only, would change the "safe harbor" rules for estimated tax payments for 2006. Under current law, taxpayers are, in effect, required to pay taxes on income, through withholding and/or estimated tax payments, more or less when they get it. But there is no penalty for taxpayers who fall short as long as their payments reach certain thresholds.

One such safe harbor, which applies to taxpayers whose prior year's adjusted gross income is above $150,000, allows them to escape penalty if their withholding and estimated payments for this year equal 110 percent of last year's tax.

The Senate bill would boost that safe harbor threshold for 2006, making people pay taxes during the year up to 120 percent of the prior year's (i.e., 2005's) tax, then let it revert to 110 percent in 2007 and later.

The effect is mainly to pull money into the government's coffers sooner -- such "revenue acceleration" is a tried and true way of making the budget look better in the short run -- but it also sets what tax people like to call a "trap for the unwary," who could get penalized.

The Internal Revenue Service is warning taxpayers about an Internet scam that uses an e-mail claiming to be from the IRS and telling them they are eligible for a tax refund. The e-mail directs the taxpayer to a link that in turn asks for personal information, such as his or her Social Security number. If the taxpayer bites, the information is used to steal his or her identity.

The e-mail claims to come from "tax," the IRS said.

The IRS, which does not ask for personal identifying or financial information via unsolicited e-mail, says if you get an e-mail like this, do not open any attachments to it. Instead, call the IRS at 800-829-1040 to see if the agency is really trying to contact you about a tax refund.

The Treasury Department issued guidance last week for provisions of this year's Katrina Emergency Tax Relief Act. Under KETRA, individuals who live in one of the four states affected by Hurricane Katrina and who suffered an economic loss may withdraw money from their retirement plan without being charged the 10 percent tax penalty that would normally apply to an early withdrawal. Such "Katrina distributions" would be subject to normal taxes, but the taxes can be spread over three years. And if the money is repaid into the retirement plan within three years, there is no tax at all.

KETRA also boosts the size of loans permitted under employer-sponsored retirement plans and provides for suspension of payments for plan loans taken out by victims after Aug. 25 or already outstanding on that date.