If more than 4gig of memory is a concern to you than I would stick with Vista x64, otherwise I would move to a 32-bit OS with 4gig of memory total.
To be honest, most typical users don't really need more than 4gig of memory, and Vista or XP 32 runs just fine with the 4gig max memory.
In the end it'll come down to funds & 64-bit support. If you have a lot of programs that you depend on for your photo needs and they won't run in a 64-bit environment, than I'd go back to a 32-bit based OS.
When I decided to upgrade to Vista x64, I also decided to invest in 64-bit software/hardware where possible. It wasn't easy, but it was the choice I chose. Luckerly I already had CS 2 before my upgrade to Vista 64, so that helped. I also have a XP Os installed on my computer as well for when I need to use a program that doesn't work with Vista x64 so that helps as well.
Also be aware that you don't need to buy the full version of Adobe Photoshop CS4 for $700, you can get the upgrade for under $200 if you have CS, CS1, 2, or 3, or another qualifying product.
Adobe Photoshop CS4: Upgrade details
Amazon's upgrade price -
Amazon.com: Adobe Photoshop CS4 Upgrade: Software
Also, did you look at the upgrade info in this link?
Windows Vista Compatibility Adobe Adobe Photoshop