Go to the source:
http://www.irs.gov/individuals/artic...=96196,00.html
This is usually pretty accurate if your input data is good.
You want to minimize your refund at the end of the year, so you can earn the interest instead of the IRS. Just make sure that you have enough withheld, otherwise you'll have a nice surprise come April 15 and have to write a check instead of getting a refund.