recompile darwin with different flags

ok boys (and girls),
I have a mystical crisis, and i want to recomplie darwin, but with a  
more aggrssive optimization flags.

something like "-O3 -mcpu=G4 -mtune=G4 -arch ppc7400 -maltivec -ftree- 
vectorize" at least. (with gcc 4, of course)

for the apple made project (so the kernel and the most important  
piece of darwin) i see that there are some fileson my hd...
what i need is some hint on what files change and how.

thankyou very much in advance, and sorry for bad english

Andrea Mannori