I just made the move a few days ago from an 8830 to 8330. I'll take a shot at this.
1. No belt strap. It did come with a pair of earbuds.
2. I want to say no, but I don't know for sure.
3. The difference in memory is very noticeable to me. I was always considerably low on memory with my 8830, but I guess I haven't had the Curve long enough to make a full assessment.
4. I don't know the exactly differences, because I upgraded to 4.5 immediately after getting home with the phone, but all of the things you mentioned are positive changes in my mind. Can't think of any others, but I'm sure someone else will know more.
5. I LOVE my curve. I never really NEED a camera, but it's nice to have and this camera takes some really nice pictures (see below). Also, I love this keyboard. Although I got really fast on my 8830, I like the feel of this keyboard much more. It's much more "clicky" if that makes sense. So, I would definitely go for it if I were you. I love this phone even more than I loved my 8830.
-Jeff-