give the kid a break guys. he didn't ask where to move to, he asked what to get.
Nate, one of the great things about AR15's is that there are about a million options for every part. this is also what makes it hard to learn about. take your time, read around the boards, you'll find the info you are looking for.
i'm not going to get into too much detail here, but, as far as uppers go, A1 and A2 both have the sights/carry handle as 1 piece with the upper. the flat-tops (A3) do not. the detachable carry handles will clamp right on to an A3 upper and have the A2 sights (the A1 and A2 sights differ).
you MAY run into a problem with the front sight post being too short with the A3 upper and the detachable carry handle. this is an easy remedy as many manufacturers make the correct front sight post to compensate for this. only problem is you will most likely pay more for shipping than the actual part.
there are a million stock options, from A1/A2, to Vltors and Magpulls. there are a million barrel length, make, and types.
best advice i can give you is to look through the AR15 picture threads once you've decided what you are going to do with the gun. find one you like, and IM the owner. ask questions.
ETA: go for something with a 5.56mm chamber. remember, you can shoot 5.56 AND .223 in a 5.56mm gun. you [usually] can't shoot 5.56mm in a .223 chambered gun.