Adobe Photoshop CS4/5 DEFINITELY 64 bit version.
In general I'd recommend 64 bit since that's "the flow".
For some legacy hardware you will need 64 bit device drivers
that will be non existant so these will HAVE to run on a 32 bit machine. Some of these might not even work on Windows 7 32 bit even in "compatability" mode.
Most 32 bit SOFTWARE that doesn't require specific hardware should run OK.
What you need to decide is what CANNOT run on 64 bit windows and how often do you run it.
If it's a small number and you are not running it all the time then create an XP Virtual Machine (or use XP mode) for these applications and hardware.
Otherwise stick to 32 bit Windows or even dual boot -- although with hardware these days much more powerful I would go the Virtual Machine route rather than use Dual Booting.