I kinda feel the same way you do. on one hand, I want DD to be the baby, on the other, I would love to give her a little sister. and if I knew I could conceive a DD again, I'd do it in a heartbeat. I think it was a very happy accident that we got pg with DD and I have a feeling the chances of doing it again are very slim. (I know 50/50, but not in my head).
I don't know if you're done. I guess if you wouldn't be totally ok with a boy, you're probably done. not willing to go HT again, you're rolling the dice, you know?
I really hate being not sure myself! I tell myself all the time I'm sooo done (especially during the summer ), but God, I love being pg and having babies. I think I will crack when DD is older and asks for a little sister (although my parents never cracked!). maybe we'll have money to go HT then so I can give her what she wants!