Collector - twins, wow! At least your DSD is older so you should be able to drive normal cars and fit all 3 car seats in... (sure that's not the first thing you thought of!)
MummyTeach - I can sort of see where you're coming from, I dont mind if I have a boy or girl next, but I do worry I won't bond as well as I have with DS, I love him so much it feels like it's not possible to feel just the same for anything else. There's also a history of PN depression in our family and often after the second DC, not the first - so it feels like this could happen this time round.
Also as a mum of a boy already, there does seem to be an expectation that I'll have a girl next to 'balance' the family (amazingly, I've never heard a mum of girls being told "I bet you hope it's a boy next"). I can't help but think there will be a disappointment from others in the family if I have a second boy. That said, I think DH would love it and I do like the idea of being a mum of two boys.
Interestingly, I think if I have a girl next I'd be more likely to go for a third, but I'd worry if I have 2 boys, if I go for a 3rd people will think (and some say) that I'm trying for a girl so I'd stop at 2 rather than have a second DS feel unwanted or 'wrong'.