One, wait until you get a picture to meet her. That's really important.
Buy and send her a webcam if you have to, but don't go to meet her without seeing a picture. Preferably, you would want to see more than one picture of her, taken at different times. The better for you to see how she truly looks.
Two, how long has it been since you two met? How much do you supposedly "know" about her? Things that sound realistic, although you can't verify them, such as her talking about her job, school, etc?
Three, e-stalk her. If you don't know them already, try to find out if she has any blogs and read them. Generally, try to find out as much as possible.
The point here is to learn as much about her as possible, to possible learn stuff that you would know if you knew her in real life, before you make any kind of decision. You can't decide simply on how she speaks to you.
Lastly, online love isn't impossible. I've known 4 couples who got together online (3 via IRC), and are still going strong. I was, and am still sort of skeptical about meeting people online, but now I believe it could work, if done right.