If your serious about building with a turbo, sizing is everything. Next is tuning, these two factors will make or break a turbo build. You can invest all you want into a camshaft or rods, but if you can optimize the tune or lag till 8000rpm your wasting. Here is a great discussion we did dealing with sizing, and I think it will show what turbos are optimizing (since Kev just mentioned it).

http://www.slantsix.org/forum/viewtopic ... sc&start=0