Is it a daily driver? If it is then I don't think you will have any trouble going with the GT510, GT525, or even the GT600 type turbos. If you get any bigger then that you will have to invest in bigger and better exaust. Meaning manifolds, down pipes and TP's. If you plan on staying with any of those turbos I don't think you will "need" a dual intake IMO. Boost is almost impossible to answer. Pump gas or race gas? I bet most will say that for pump 14-17 psi is pretty safe (assuming you motor is running properly) and should yield pretty good power with minimal lag characteristics. If you just want a little bit more power then stick with another 4 bolt turbo like the GT510 or GT525 and the like.