Hmmmm.
So your question is:
What will it take to get 500hp?
The general rule of thumb is 1.0 Bar will yield 100% the NA horsepower roughly.
So, 1.0 Bar = 14.7 psi
Naturally Aspirated 318 with 9:1 CR or less...would be required to make 250 hp before forced induction to attain the hypothetical 500hp on 14.7 psi.
If you want to run a lower boost level (say 8-10psi) and make the same power your 318 would have to make more power before boost to be able to reduce the boost level and make the same amount....understand?
Also, the ignition timing and fuel system would have to be upgraded to maintain the correct timing and volume for the newly added Air Pump.
The best place for you to start with your quest is:
http://www.theturboforums.com/smf/index.php
Then, get the Book : Maximum Boost by Corky Bell
Then bust out the check book, and your dream is as doable as the $$$$ you throw at it.