Question: What's are the advantages, if any, of PictureProject over iPhoto 4?

PictureProject works with Nikon NEF (RAW) images for all of Nikon's cameras.
Keyword editing and organization - PictureProject lets you assign keywords to images and also sort images by keyword. Incidentally, this will work seemlessly with Tiger's Spotlight technology.
PictureProject lets you organize your pictures into folders and subfolders, iPhoto 4 doesn't allow you to create subfolders (though iPhoto 5 is supposed to add this).
PictureProject has a Panorama maker for stitching multiple images into a single, large panorama (though it looks like a bad Windows port).
PictureProject supports non-destructive image editing - the original image is always preserved no matter what you do to it.

3. Unlike iPhoto, when you make adjustments [brightness, effects, etc] the original file is changed, not a copy of it.

This is both true and false. PictureProject always preserves the original image data. The way it does this is by saving the modified images in the same file as the original image. So if you have modified an image both the original image data and the modified image data are saved in the same image.

IS the Mac version the same as the PC?

Yes, the Mac version is pretty much the same as the PC version.