Photoshop CS3 beta is out for the Mac and is much faster (for me anyway) than CS2. As to whether or not you need it, that all depends on what you're trying to do.

Elements is still not a Universal Binary, IIRC, and will probably run pretty slowly until 5.0 for Mac comes out.

Lightroom is a great program, though IMHO, it is missing a good builtin backup app much like the one in Elements.