Chris@VTT put together a good article on getting your N54 to the ~600 rwhp level. Check it out below.
While I think most of the users here on BB are a little past this, but I wrote an article about how to get 600 whp out of your N54 powered BMW. It's not a DIY but goes over the basics of what it takes to get there from our perspective -I hope some of you find it useful. Screenshots and .pdf attached.
I do want to reiterate, there are MANY ways to do this, and plenty of good quality parts out there, not everything that's good got mentioned. If you have questions, just ask, we're always glad to help.