My CS4 package arrived yesterday, and I installed last night. Since I didn't fully understand how PS is implemented in a 64 bit environment, I thought I'd share my experience so others in the same situation can plan.
CS4 installs two completely independent instances of PS, one in the 32bit program library, and the other in the 64bit. You get two entries in All Programs, one for Photoshop CS4, the other for Photoshop CS4(64bit). Each is an independent installation, with its own preferences, etc.
Third party plug-ins won't work in PS64 unless the vendor has written a 64bit version. In my case I can't use Noise Ninja and Genuine Fractals in the 64bit version of PS. 32bit works fine.
Twain drivers won't work in 64bit. In my case, I can't import from my scanner in 64bit. 32bit works fine.
I have yet to figure out a workflow with CS4, but until I can get the 64bit plug-ins I use most, the 32bit version will carry the load.
I did a search, but could not find any previous posts covering this, so if this has been covered before -- oops.
Hope this helps those planning an upgrade.