The XAMPP installation is entirely contained inside the directory:
There are no file conflicts with the existing Apache web server, or PHP that come with OS X 10.4.x
However, they are both configured out of the box to use the default port 80, so running both at the same time will result in conflicts. As long as you disable the OS X Apache server (System Preferences > Sharing > Personal Web Sharing) you will have no problems.
The nice thing about this installation is that it is entirely included in that xmapp directory, so you dont need to worry about searching for config files in obscure places on your machine!
You could modify the configuration for one of the apache servers to use a different port, and run multiple web servers on different ports.. but you are probably better off just running one at a time.