In terms of boost, the components that will take you from high 200whp to high 300whp/low400whp are very small in terms of cost difference. Your main concern is reliability. For example, leaving the stock fuel pump vs replacing with a walbro 255 is ~$80 and well worth the upgrade. Injector sizes are typically close in price, for example 650cc injectors vs 1000cc from FIC/ID. It'll run you ~$400 and that's without a fuel return. FlashPro is necessary for either setup, as is a custom tune (I suggest E-Tunez). Stock fuel rail is perfect, doesn't need to be changed. Blow off valve, intercooler piping, intercooler, and the like will remain the same regardless of power. The only real difference between low boost and high boost (6psi vs 16psi) will be your turbo/manifold/downpipe choice.
My advice, based on my and others experience, is to buy a used manifold and downpipe. Good luck getting a used cast GReddy/TSI manifold, and the GReddy manifolds are known for cracking and GReddy won't replace them. It's better off to look for a used tubular manifold (Full-Race, SRT, JoTech, AJP, etc) and downpipe and then buy your turbo brand new.
Give us some more details on what you're looking to do and your current setup and I can elaborate more and make a solid list of parts you'll need and approximate prices to make your dream come true. As a side note, I have a 400+whp Daily Driven Si and she's in great shape. If you buy legit parts, you won't need to worry. There are 500+whp and 600+whp daily driven rsx's around without a problem.