If you have to ask the question of 4 vs 8, the answer is no. You don't need 8Gb.
As said, apps like Photoshop CAN use lots of ram if you are doing stupid things with it. I've ran 8Gb on my Vista 64 and W7 system for quite some time and while i do occasionally do stupid things with Ps it will use more than 4Gb, but mostly, its just a waste.
OSX on the other hand will happily use anything you give it. In the past my Mac mini's have been happy enough on 1Gb, my iMac in also happy enough using over 7 of its 8Gb.