I am a former HR manager and I gave this speech all the time... put in the maximum amount that your employer allows!

The benefit is more than what you will have in 20 years, it's also the tax savings of your investment being non-taxable.

The performance of your 401k depends upon where you put your options, so there is no way to say exactly how much you will have at the end of 20 years.

From my own experience, in one year alone I put in about $4000 and my employer matched it with 2% common stock. The common stock went NUTS that year, split twice, and I made over $10,000 in about 8 months!

On the flip side, since 2001 I have only broken even... losts lots of money in some funds (like merck and energy providers) then made it back on some high risk oil and construction. We were losing so bad that we converted to cash holdings for a while. We recently cahsed out what we had in it from 2001-2005...about $18,000.

It's important to look at the BIG PICTURE when investing in a 401k. Start now and put as much as you can into it. You'll see the return fluctuate through the years but you can't count on that . What matters is what you have when you retire. It'll all balance out and you will see a profit.

be sure to balance your funds between low and high risk. Being younger, you can afford to take a chance on the higher risk funds. Your annual prospectus will show you the trend of how the stock has performed over a period of time.

Yeah, the first answer is a good one. Another company that is excellent and has low-cost mutual funds and other investments is found at www.vanguard.com. You'll be very well off if you do as you plan and invest wisely. Look for investments with reasonable returns over the last ten years that have low costs associated with them because any fees or commissions they charge come right out of your pocket.

Don't forget you have other option available to that many help you grow your money. I'm speaking of the Roth IRA if you put the same money into the Roth IRA as you do the 401K and invest in the same funds, when you retire at let's say age 60 both accounts should be at the same amount in monies(lets say $1,000,000). Here is the big different now, when you with draw your money from the 401K you with be taxed on that amount so the $1,000,000 is now worth about 650,000 after taxes. When you remove the money from a Roth IRA it is Tax Free you get to keep 100% of the money so the $1,000,000 is still worth $1,000,000. So right off the bat at age 60 the two account differ by $350,000 because of taxes. ( 35% tax bracket)
If you employer is matching your 401k and it is less then 25% you will better off with the Roth IRA first. You are limited to how much you can put into a Roth IRA 4000K each year , currently $4000. After you have max. out the Roth then you can look at the 401K . 401K are good investment tools but if there is something better you should look at it and check it out.
Again Fidelity.com has a great website for any question on IRA's and 401K and other retirement question such as retiment calculators, just type in the search box which one you want more information on.

The best thing is that you're 21 and doing this now. That will return volumes when you retire.