Smarter operating systems like, Windows 7, will analyze your usage patterns and try to schedule programs to put in to memory so that they load quicker, for example (SuperFetch).
RAM sitting idle isn't doing anything for you. Likewise, an OS that mismanages it, or doesn't have enough is bad too.
So the answer is "it depends". Put in 6GB and you might find the OS will naturally occupy around 4GB on average...but that usage number might not change if you up the RAM to 8GB. In your case, with Photoshop, I'd put in as much RAM as possible. And this assumes you're running a 64-bit operating system.
I'm coming from a place where for the last 10 years I've been running an old Dell P3 512MB RAM ATI Radeon 32MB GPU machine. I never loaded anything memory intensive on it because of the hardware, so don't have a great grasp of how much memory is going to be enough to make it "fast" (and feel fast for at least 5 years), and how much is a waste of money.Trying to understand how the OS makes use of memory...so even though Win7 and a few running programs may only
need say 3GB quickly, an additional 3 GB, (or 5GB if I had 8 total) would contribute to making
everything run a little faster, rather than "hitting the wall" where the extra RAM doesn't get utilized, like if my OS were only 32bit? I read up on Superfetch a little. An interesting feature, and sounds like something that definitely likes more RAM.
This is my work computer, so I'll never do any gaming on it ;-). The version of Photoshop I use is Elements, V7 (for now), and don't
think it's as big of a RAM hog as CS5. I do some photo editing, but haven't taken the time yet to learn all the bells and whistles on PSE to do serious stuff. Some of the scans I edit are high rez, so files can exceed 10MB easily. I am planning on loading QuickBooks and some kind of vector conversion program to work with converting old lettering books into usable font libraries. I may do a little video capture/encoding of Mini-DV tapes, but trying not to make this computer my primary place to do it. I hope to have enough money later this year to build my own for home and do most of the video stuff on it.
I do run XP mode on occasion (my OS is Win7 Pro 64), but that doesn't seem to be too hard on memory usage so far.
Whew, sorry to be so long winded!