They have some what of a correlation...but basically it works like this:
Theoretically if you can get a 5 on an AP you can get a 750-800 on the SAT II, and vice versa. The AP is easier on the scoring in a sense because its just 1-5 and you dont know how the raw scores calculate into the scale.
The SAT II is much easier to look at in terms of scoring...but i would assume a 750-800 is like a 5 on the AP, where as like 690 - 740 or something is like a 4. (these are just made up calc's i cant tell you if there even accurate at all)..
But yes the trick is to do well on SAT II's is to take your best subjects and which ones you are most comfortable with.