layouts CSS files

Hi everybody !

I never tryed to work with the layout CSS files located in the zen layout folder...
Always with the layouts-fixed or liquide CSS files located in the CSS folder.

But I wonder :
If I want to buid a 'one sidebar first' website, should I works instead with all CSS files located in the 'one_sidebar_first' folder instead ?

If so, which files should I put in my sub-zen folder ?
Which files should I override/replace ?

Thank you for your help in advance...