Upgrading or tweaking hardware just to run Photoshop that bit quicker has become an obsession for many people - and if you're one of them, I came across the very interesting blog by Scott Byer, one of the principal architects of the application.
http://blogs.adobe.com/scottbyer/
Check out his PDF presentation on Photoshop Performance (Sept 12th 2007 entry) - and it's also worth browsing through his responses to the post about a possible 64-bit Photoshop (Dec 20th 2006).
As a test I'm currently running PS CS3 under Vista 64 bit on a system with up to 6GB of RAM and will report back with any interesting findings. It may be a 32-bit app, but there are still benefits to running it under 64 bit Vista with lots of RAM...
Gordon