The 9550/9600SE are out classed by the older 8500/9100 series. For example, the 9550 has a core clock speed of 250MHz and a memory bus speed of 400MHz. Whereas the 9600 Pro has a core clock speed of 400MHz and the memory is communicated with at 600MHz. Other than that, the Pixel Pipeline, Texture Pipeline, and Vertex Pipeline are the same. All other things being equal, I would prefer the higher speed card for gaming (the 9600 Pro also comes in a 128 MB version).
One thing to keep in mind, if you're going to be playing Open GL games, like Doom 3, then nVidia would be your choice. They have done a better job with this feature than ATI has with their most recent cards.