just installed osX on top of darwin, here is the report:
there was a customized install option, but I was unable to uncheck anything, so just proceded with the standard install (not clean!)
as I expected, it kept all directories that were there, integrating itself with what already existed - since I had installed php, apache, configured network settings in darwin; all that stayed - I still had to tinker with some settings but overall it was quite an easy installation...
moments after installing, I started up apache (the one I installed in /usr/local/apache) and was serving webpages!