Sometimes, I run PS, LR, and Photomatix all at the same time processing RAW photos. I only have 4 GB, but I'm able to get the job done. I want to expand, but I'm not sure how much more RAM I should get.
If you have a 32bit OS then 4GB is your limit - anymore and you're throwing money at a lost cause.
If you have a 64bit OS - chuck as much money as you want at RAM. Depending on how old your system is, 6GB on DDR3 triple channel memory spec machines is a good minimum to start with and work up from there.
It's not just the RAM though, a good processor will also aide in the process(es), but again depends how much money you want to throw at it.
8 GB ram each on MacBook pro and Ubuntu Linux laptop, but would love 16 GB for my linux laptop. On my Mac, I use viewNX, Photoshop Elements and been playing around with Aftershot Pro and Darktable because it runs on both operating systems. On Linux I'll use Darktable and Aftershot Pro.
I currently have 16 gig but thinking of bumping it up to 32 in the coming months. Passing around these 36 mp images between LR, PS, Nik and Photomatix, chews up a lot of memory. Fortunately I'm running the third gen i7 processor so there's more than enough engine to pull the load, I just need a little more room to park the train between stations....if you will.
I do fine running 8GB under Windows 7 (64 bit) and using CS6 but then I don't have extraordinary processing needs, either. Photoshop can be pretty demanding on your system as a whole so a good processor is equally important; all it takes is one bottleneck and the whole system is slowed down.
8 seems to be more or less standard now. Anything less and things start to slow down. For Windows users you really need to get a 64bit machine. Not sure why they even still sell 32bit.
4GB on my imac and 6gb on my laptop. I only use my mac whenever I have to edit. Even though it's painfully slow when exporting 10-12mp edited photos, it's still good enough for the overall workflow.