401K is a retirement plan.
What is thebest investment option for a young, single professional?
Do: Buy a home you can call your own. There will never be a better tax write-off than a home. Next, contribute into your company's 401k if the company matches their contribution, but only up to the point of the match. Finally, contribute to a Roth IRA (if you qualify), or a nondeductible IRA if you don't qualify for the Roth, so you can take advantage of the Roth conversion possibility in 2010. However, before putting money toward any of the investments above, you MUST pay off any and all credit card debt you have. Getting out of debt is the most important priority before investing for retirement.
... never ever borrow against your 401k plan because you will pay double taxation on the money you borrow. Because you don't pay taxes on the money you put into a 401k, when you pay back the loan (which you must do within five years, or 15 years if used to buy a home), you pay it back with money you have paid taxes on. Then, when you retire and take the money out again, you end up paying taxes on it a second time. And that isn't even considering the penalties you have to pay if you change jobs/quit/lose your job, in which case the money is due immediately and subject to taxes and a 10% penalty.