20 psi through what? 2871R? 35R? 42R?

Running 'such and such' boost is such a relative term. The goal you need to figure out and answer first is HOW MUCH POWER ARE YOU SHOOTING FOR. Once you can come up with that answer, deciding between an SR20 or KA-T will fall into place, as well as what the build should consist of, and what turbo to select.

Ryan posted some good info.

And for Christ's sake, don't do like most 240 owners do. Get a true EMS for your setup. Don't skimp out on the one thing that will keep your motor in sync.