when it comes to turbos your first question should be how much HP you want (goal) then you go over to turbobygarrett.com and read their turbo 101,102 etc. you will learn how to read compressor maps and understand turbos better, you can then apply the a/r etc to similar turbos to meet your goal if you're not into buying good turbos, although not all turbos are built the same and different manufacturers may/may not have maps for their turbos. also remember your motor (b20) won't handle tons of boost with stock sleeves and although boost numbers shouldn't matter (yet)keep in mind you'll be shooting for 6-10 psi approx 225-270ish HP. you'll probably be looking for something in a 50-57 trim.
oh and also, you will want a turbo that matches the flange on your manifold, unless you plan on getting a new one if the one on your new turbo is different.