Oh hun, big ol' hugs. It's a tough place. I went for #3 and didn't end up with my desired gender (I have 2 DD and am expecting my 3rd ... I always wanted boys) and I won't lie, it's been extremely hard. Like you say, of course you'd love another boy ... I will adore this next daughter ... but I'll still have GD. It has gotten progressively worse with each DD because it just feels magnified by each additional child that isn't my desired gender. I know I don't have to explain that I absolutely adore my girls, I wouldn't trade them for all the boys in the world just as I'm sure you'd never change your sons. But my heart has always wanted a son and out of 3 children, I didn't get one. It's a longing that will always be there :(
So it is a roll of the dice, but unless you try you can't get what you want .... at least by trying you have one more shot. You could have your girl you know! But if it doesn't turn out how you hope, with a DD, it still yields another brother for your boys to love, another child for you to adore ... it's still a win, really, but the cloud of GD can make that very hard to see at times.
Re: the age gap and your DS2 being almost 5, I did mention it on another thread but it can be so sweet to have another baby around after a gap. Your boys will have such a neat experience, getting to really teach her (FX!) things. All the firsts like smiles, giggles, etc will feel so fresh to you again. Yes, it is a bit of a pain to go back to diapers, etc, but really... time flies. Your #3 will be in school in no time and you'll wonder where it all went, KWIM :) And in the grand scheme of sibling relationships (since they'll be adults MUCH longer than children!) 5 years is nothing!