The short answer is that any contribution into your 401k will decrease your take home pay. However, the amount that your take home will change depends on what your marginal tax bracket is.

To figure the change to your take home pay, add your federal tax rate (should be 10%, 15%, 25%, 28%. 33%. or 35% depending on your filing status and level of income) to your state income tax rate, and multiply that times the amount you want to contribute to your 401k or HSA. Then subtract that figure from the amount you want to contribute to derive your change to take home pay.

In 2010 the tax brackets for single filers is:

0-8,375 10%
8,375 - 34,000 15%
34,000 - 82,400 25%
82,400 - 171,850 28%
171,850 - 373,650 33%
Over 373,650 35%

For example, a person who files his or her taxes as single, with an adjusted gross income of $60,000 will generally be in the 25% tax bracket. Typical state income taxes average around 5% on that amount of taxable income. This means that 30% of his or her income goes to taxes. If nothing is contributed to 401k or HSA, then federal and state taxes could eat up as much as $750 per paycheck, if paid twice a month. If, however, this person contributed $100 of that income per paycheck into a 401k or HSA, then they'd only see a decrease in take home pay of about $70.

Keep in mind also that your employer may sweeten the deal by matching some of your contributions into your 401k or HSA. So even though you see a drop in your take home, if you can afford it, you'll see your savings grow quickly.