I wouldn't buy one online. There's something to be said about a professional fitting and a test-ride. The stem on the bike I wanted (LeMond Reno, like 4-5 years ago) at one of the local stores was too long, so they got me a shorter one and I was all set. The dude took out the weight-on-the-string and did his thing to fit it properly.
Find a place which sells a lot of road bikes and find a salesperson who knows his/her stuff. It worked for me.