If memory serves me (and it has been a while...), I think the 1600-A is the form you use if you are filing before you have identified a specific child (or children). You use the I600 if you already know exactly which child or children you will be adopting. If you pay the fee for the I600-A, you will need to fill out the I600 once you know who the child is, but you will not have to pay another fee. You pay for either the I600 or the I600-A, but not both.
For anyone new, who might ask why bother to do the I600-A rather than just wait til you know who the child is...it's because getting the I600-A approved can take months, so if you go ahead and file, and get approved while doing other things (like sitting on a wait list for example...) you'll be ahead of the game. Converting to the I600 takes no time at all as long as the I600-A has been approved.
Now hopefully that is all right--I'm going by memory here.
Teranga