A 32-bit application still has a 32-bit address space, so it cannot make use of more memory, unless it uses the "address window extensions" API - so you need a 64bit version of photoshop to take advantage of extra memory.
For 32-bit windows, there's boot.ini switch to change the usermode:kernelmode memory address space from 2:2gig to 3:1gig - you should google the boot.ini /3G switch for the implications. Notice that windows will *still* use the default 2:2 split for applications that haven't been marked "large memory aware" though!
Also, even regular 32bit windows should be able to take advantage of >4GB memory, as long as your processor supports PAE mode (introduces with the pentium pro a zillion years ago) - it's turned on by default if you have DEP enabled. This won't let a single application use more memory, but the physical memory can be used for separate instances of your apps, system cache, etc.