Bit of an unlcear title there, but in any case as has been covered by BareFeats.com, under Tiger CS2 apparently offloads the RAM caching to the OS, and it is capable of using the full available 8GB (even 16GB on an XServe, I suppose). Check out their article:
http://www.barefeats.com/cscs2.html
Basically, CS2+Tiger (and only that combo) taps into some new, unlimited caching algorythm, even if you don't ask Photoshop to (Motion apparently does something similar). The upside is as much as a 5X speed increase on certain repeated operations.