The issue is not having enough RAM to run Photoshop, it's having enough RAM to work on the size of document you have open. If you're only working on small, lo-res images you won't need a massive amount of RAM. If you're working on hi-res, multi-layered images, you'll need a lot.
I've heard it said that you need a minimum of 3 times the document size in RAM. In reality, I think you'll find you need more than that for it to run smoothly.
If you think it's running slower than it should, make sure you check how much room there is available on your hard drive(s) too. Photoshop runs like a dog if your drives are full.
I hope this is helpful.