NEW YORK (CNN/Money) - The numbers could be huge: The higher contribution limits and tax-free withdrawals that are part of President Bush's proposed retirement savings overhaul could boost your bottom line by hundreds of thousands of dollars.

But -- and this is a big but -- that assumes you're part of the small percentage of Americans who can afford to sock away an extra $15,000 a year on top of what you're already saving.

Here's a look at some of the math behind those proposals and how the changes, if Congress approves them, may affect you.

The centerpiece to Bush's savings plan overhaul is the creation of two tax-free accounts: a Retirement Savings Account (RSA) and a Lifetime Savings Account (LSA). Instead of making pre-tax contributions, getting tax-deferred growth and having to pay income tax on your withdrawals (as you do in a deductible IRA), you would make after-tax contributions, and enjoy tax-free growth and withdrawals in an RSA and LSA.

What's more the contribution limits would be different. Under current law, you may contribute up to $3,000 a year to IRAs (with scheduled increases up to $5,000 by 2008). Under Bush's proposal, you'd be able to contribute up to $7,500 a year to an RSA and to an LSA, for a total of $15,000. (That limit would rise with inflation.)

And unlike with IRAs, there would be no income limitations on who may open an RSA or LSA, nor would there be any restrictions with the LSA on when you take the money out or on how it may be used.

All that can add up.

The Bush administration wants to overhaul the country's retirement programs. CNNfn's Valerie Morris takes a closer look. Play video (QuickTime, Real or Windows Media)

For example, let's say your combined federal and state tax bracket is 30.65 percent (and will remain so when you retire). And let's assume you earn an 8 percent annual return in an equity income fund. If you contributed the maximum allowed to a deductible IRA every year for 20 years, you'd net $151,210 after taxes, according to T. Rowe Price.

(That figure would be higher if you faithfully reinvested your annual tax savings from the deductible contributions in a taxable account and earned an 8 percent return every year. Then the net take-away, combining your IRA and your tax savings, jumps to $204,748.)

But under the president's proposal, if you were able to contribute $7,500 a year in after-tax dollars for 20 years to an RSA, you would have $370,672 upon withdrawal. If in addition you were able to save another $7,500 in a Lifetime Savings Account for those 20 years, you'd have double that -- or $741,344.

(Actually, you'd have a little more, since the $7,500 limit will be indexed for inflation, but that was not figured into these calculations.)

Still, for most Americans that may be wishful thinking since most people don't take full advantage of their savings options even now.

The Investment Company Institute and the Employee Benefit Research Institute (EBRI) found that of the 31 percent of households with tax-deferred IRAs, about 80 percent of them did not make a contribution in 2001.

"Since only a very small minority are at the maximum now, it's very hard to see how raising the limits would result in a behavioral change for anybody beyond that group," said Jim Jaffe, an EBRI spokesman.

If you do have a deductible IRA and have come to rely on the immediate tax break it offers (your contribution reduces your taxable income), then that benefit will be removed because all IRAs would be phased out.

The President has also proposed to Congress that a host of workplace retirement plans (including 401(k)s, 403(b)s, some 457s and Simple IRAs) be consolidated into one type of plan -- the Employer Retirement Savings Account (ERSA).

The ERSA would function much like the 401(k) except that a number of the rules governing the ERSA would be different. And the changes, in effect, "create the potential for highly compensated people to defer more money" than they can in 401(k)s, said Ed Ferrigno, vice president of the Profit Sharing Council of America.

As it is now, people making $90,000 or more may be limited in what they can contribute to their 401(k)s if the company's lower-compensated employees don't make contributions above certain levels. The rules governing those levels have been relaxed significantly for the ERSA.

The changes will be positive for high earners, but will not negatively affect lower-income workers in the plans, Ferrigno said. Indeed, the ERSA will be the account to use if you wish to reduce your taxable income and you want to benefit from the employer match -- which is essentially free money that automatically boosts the return on your investment.

With the exception of high earners looking to put more money in tax-deferred plans, Bush's proposals are "unlikely to have a material effect on employees' decisions to save," said Lori Lucas, a defined-contribution consultant at Hewitt Associates.

Hewitt has found that those who don't take much if any advantage of their retirement savings options at work are primarily the young, the low earners and those with very short tenures at a company.

The reasons for their low savings rate are typically that they feel they can't afford to, they want quicker access to the money, they prefer the immediate gratification of a bigger paycheck, or they lack understanding of the impact of compounding on one's long-term savings.

But, she noted, Bush's proposals may encourage some workers, especially the younger ones, to save money in the highly flexible LSA and if it gets more people to save, that's a good thing. Still, she added "it's a double-edged sword." That's because the LSA -- with its promise of anytime, penalty-free withdrawals -- may make people more short-term oriented in their savings and it may keep some people from contributing to an ERSA beyond what they need to in order to get their employer's match. And that can have a negative impact on workers' retirement savings.

Nevertheless, experts agree that the tax-free LSA would be a boon for savers at all income levels who might otherwise sock savings away in taxable accounts.