I run PS CS-4 on my G5 dual and still run my 9800 on the platform.
If you need to open a raw file on from a newer camera than your version of Photoshop supports, you can probably run it though the DNG converter to make it backward compatible.
Since PS CS-5 has to be "dumbed-down" to run the Portrait Packages and Contact Sheet II, I still keep CS-4 going for this very reason. It's a PIA to do contact sheets through Bridge or have to shut-down CS-5, set to 32-bit and reopen only to have to repeat the process once you are done. Both those are such time savers and worked well that I'm surprised they haven't been updated to 64-bit.