Forseti, when you say "plugins" do you mean additional programs for Windows or for Photoshop?
If its photoshop, then the version of windows is irrelavent, as installing pre-CS4 will just install the 32bit version anyway, and CS4 will install both 32 and 64bit versions - and you can run either - I tend to use CS4 64bit on my Vista 64 machine, because I can, and only the 32bit version if a plugin doesn't work.
If its windows, then I've not had a problem if the application is only 32bit - it may run a bit slower as the OS has to emulate a 32bit environment, but can't say its noticable!