http://travel.howstuffworks.com/rotary-engine9.htm
Always an exellent place to learn how stuff works..You'll find everything there is to know about rotary engines there.
http://www.fasterfords.com/mazda/rx7.htm
A nice little chart you might find useful. Statistics about the 1st gen, 2 gen, and 3 gen RX-7s...blah blah..
Im pretty sure they dont make Turbo kits for them..but I have heard they make supercharged kits..but for the amout you have to pay..which is frikken ridiculous..I dont think its worth it.
What else...Turbos don't last as long as N/As do. If thats what you mean by which one is "stronger" Turbos around 150k..if im not mistaken...and a well cared for n/a will get you over 200k. But as far as performace.. TurboII > n/a