Perhaps it might help you to analyse the reasons why you would have preferred a girl, and why you think elder boy/younger girl is the "ideal" family?
Because I bet if you think about those reasons carefully, you'll find they don't really stack up in reality.
For example: is it because you have been led to believe by films, adverts, friends' comments that "one of each" is the ideal family? If so then let that go - it's just a social cliche, it doesn't have any basis in reality at all.
Is it because you think a girl will be easier - trust me, they are not necessarily, it all depends on what kind of boy/girl you get.
Is it because you wanted "something different" - well you will have something different, your DS2 will be a totally different person to DS1 (in fact he may be more different than a DD would have been, except for his sex).
It is worry about 2 boisterous boys - your DS2 may not be boisterous. Your DS1 may grow out of it. If you'd had a DD she might have been boisterous (I know plenty of boisterous girls).
Honestly any reason you come up with probably isn't borne out in reality (unless it's that you wanted to buy dresses - but that's pretty minor
)
I have 2 girls and for a few moments I was disappointed that DD2 wasn't a boy. Especially as others I knew were having "one of each" families. But I pretty quickly got over it by applying the technique above 