I wouldn't find out if it were me.
With my first I had my heart set on a boy and decided not to find out, and am very glad that I didn't!
It meant that finding out what the sex was wasn't an issue in amongst all the other emotions that I was feeling at the birth, if I had been in a sound state of mind when finding out I may have had a resentment of sorts (not a good thoguth to have but you have to look at it in the way of time, from your 20 week scan you will still have 20 weeks to go, will you be able to come round to the idea, I knew that I wouldn't but everyone is different!)
It comes down to the type of person that you are, if you are strong enough to bring yourself round to the idea of having another boy then it's up to you.
Also, is it really THAT necessary to know what the sex is?
We decided to find out this time because we needed to know whether or not to sell the clothes etc that we have left form last time, luckily it looks like it's another girl so we can just keep the stuff we have, but most people don't have this problem and just find out anyway, I would've preferred not to know, it's just not natural, and still not 100% anyway LOL
Hope that helps, sorry if it doesn't LOL, I blame the baby brain