The real question to answer first is can people find your house. If you live in a well known area, with lots of drive-by traffic, then do a for sale by owner. But if you live in a great location but on a street that no one has ever heard of, then you'll be wasting your time with FSBO. The very first thing most people think about when buying is the location. If they read an ad in the paper and don't know the location, forget it they'll never call and they'll never buy. That is when a real-estate agent helps. He'll get his coworkers to show their clients at your house even if they've never heard of your street. I sold my house in a short time with an agent while my FSBO neighbors wasted 6+ months for the same house on the same street for the same price and the same condition at almost the same time (our street was only 1 block long and no one knew where it was). They later got an agent and it sold in weeks.
The next question is what is the house truely worth. It isn't worth what you want to get. It isn't worth what you paid for it (or even some percent above or below). It is worth what buyers are willing to pay for it. If you can judge this price properly, then you might not need a professional's help. But if not, you'll be sitting there 12 months from now wondering why you just paid $20,000 in interest, maintanence, and taxes instead of paying a realtor $10,000 to sell the house quickly.
Finally, is your house in selling condition? I can't count the number of homes I've seen on walks with a FSBO sign and a yard that looks like crap. I'm talking piles of trash that would take only 10 minutes to clean up (or similar issues). Say goodbye to any sales if that applies to you. An independant real estate agent can really help give an unbiased eye. And they often provide professional house stagers for free to help move furniture around and remove eyesores that you'll never think about.