8gb will be more plentiful resources to do multimedia & gaming purposes.
There should be only a little gain over performance, but, it will make you able to run much more apps at the same time. Like running Photoshop, encoding video, and running Crysis 2 at the same time.
Hmm, if you can plug the memory separately after, I suggest you go with 4 gb first to see if it is adequate enough for you. If later you need more, you can buy this another 4 gb.