Even for most games 4 GB is enough... 6 the most you'll see an appreciable benefit.
But, if you have only have the 32 bit version on the OS, you can only have 4 GB of RAM anyway.
Some heavy photoshoping, video editing, ect. you'll need more than 4 or 6 GB of RAM. But chances are, if you're doing that kinda thing, you already know about it