Girls apparently tend to produce more hCG (don't quote me on this but I think I read the other day there have been studies done which show this), so you're more likely to experience more severe morning sickness with a girl.
Having said that, plenty have it with boys, and let's not forget the possibility of multiples!
There's a whole list of things I read that apparently can help you lean towards one or the other, I shall have to find it and post it.
With our boys I didn't have sickness, I had maybe a day of it because I was hungry, but with DD I was so unbelievably nauseous! With this one I have been the same as DD but more so and for longer. With DD my worst week was week 7 I think, it started to calm down after that a little but this time it won't go away.
Someone on here posted extra B6 can help with nausea