enid, I didn't find out for dd or ds1, and DID find out for ds2 because my dh's father was dying, and I wanted him to know what his next grandchild would be. so found out on the tuesday, told him, and he died on the sunday.
the strange thing was that I really thought that knowing the sex would make the whole thing a bit more pedestrian somehow - not having that exciting moment of finding out at the birth - but it didn't at all. It really didn't make very much difference. Except that we could manage the expectations of dd,who was DESPERATE for a sister, by saying "well we aren't sure, but we THINK we saw a willy, and if we did, it will be a boy". By the time he was born she had got used to the idea of a boy, and was fine with it.
This time around we thought we might as well find out again, again partly because dd was in agonies about wanting a sister (or horrified at the thought of maybe having 3 brothers) - but also because it is DEFINITELY my last baby and I wanted to clear out my loft of either girls or boys clothes! Anyway it is quite nice knowing, and being prepared, and waiting to meet her. And of course dd is thrilled that she will be getting her sister. But if it had proved to be a boy, I think it would also have been quite useful to know in advance to prep dd and to deal with the slight disappointment of not having another girl (which I wasn't too bothered about but you know...)
So this is a very long way of saying that if it matters SO much to your dh I think it is better to find out now. If it is a boy it will be a fantastic fillip for you all, and if it is a girl your dh can get over the initial disappointment and then start looking forward to meeting her, wondering which of your dds she will look like, etc etc....
Hope the little thing doesn't have his/her legs crossed after all this!