staryeyed, I was the same as you and now have four boys! I didn't find out the gender on any of them but ds4 and I have to say, for me it was the wrong thing to do. I spent my third trimester crying my eyes out. It didn't help that my SIL was expecting and had a girl and called her the name I was going to call my dd ( not SIL's fault, we hadn't actually discussed names and in any case she could call her dd what she liked!)
But when ds4 was born, I did fall in love with him. He was different from the other boys, full head of hair when they had been bald, right handed instead of left handed etc. He is his own little self and is a fantastic little person. When he is naughty he says "Sowwy Mammy!" in the cutest way and he is my little angel.
I am still sad that I don't have a girl but I do think this is easing for me now ( he is two) I think what helps for me is looking at other peoples children and realising that little girls are not always cute little rays of sunshine, sometimes they are untidy, messy naughty little things just as my boys don't necessarily fit the "boy" stereotype.
By the way, it is always 50:50, no matter how many you have of each gender.
The website mentioned above by beeteee is probably in-gender.com and is very helpful. I have found the "Rising above gender disappointment" forum very helpful.