Now, it's none of my business how many bits you process at a time. That's between you and your software vendor. But, if you want to continue using Adobe Photoshop on your Mac, you better jump on the upgrade train and grab a new multi-core Mac that has a 64-bit os, Snow Leopard or later. Windows retains 32-bit support, but only for Windows XP.
Part of this change in support structure is Adobe dropping support for Windows Vista. The logic behind retaining support for Windows XP is simple: many Photoshop users are still on Windows XP based system. With this update, any existing 32-bit Photoshop plugins will need to be updated for 64-bit. The requirements for the current beta are: