Just to mention it, Photoshop CS5 x64 still comes with a 32 bit version. ; So long as the plugins are still basically compatible with it you can continue to use older 32 bit versions as long as you're running the right version and have the plugins in the right place.
As to how to access the 32 bit version, in windows you just run the 32 bit version that installs into the "program files x86" directory instead. ; I mean it'll be nested under it, then under adobe, and so on. ; But ends up under the x86 version instead of just "program files". ; You also have to make sure that the plugins files end up in the file structure for that version of the program.
Macs seem to handle it a little bit differently, there's something about a "run in 32 bit mode" option you get by viewing application properties. ; But as I don't use macs I'm unfamiliar with where you actually find that.