Photoshop is one of the biggest consumers of RAM there is. If you do a lot of Photoshop work, no matter how much RAM you have, it's never enough.
However, having said that:
The delay you see with viewing large thumbnails in Bridge isn't related to RAM. It's related to the speed of your hard disk. The previews are generated by looking at the image on the hard disk; since a hard disk is literally thousands of times slower than RAM, you'll get delays no matter what. You will see a performance boost if you replace the hard drive with a faster model. You'll see a BIG performance boost if you replace the hard drive with a SSD.
It used to be true, in the ld days of OS 9 and before, that if you had more RAM than you were using, it was wasted money. The Mac used to use a static memory allocation scheme--you had a pool of RAM, you told each program how much RAM to use,and RAM that was not being used just sat there.
With OS X, the operating system will use unallocated RAM for buffers and such, so it is being used. You might not notice a huge performance boost, especially because some of the boost is masked by having a slow hard drive, but it is there.