No, OCing is not memory dependent. 4GB should be fine. That's all many people use.
unless your using ridiculous memory intensive stuff 4 gb is fine, most of the time i never use more the 2 or 3 gb when i have a ton of windows open. Unless i'm using a program that intentionally uses as much ram as possible 4gb won't be a problem
So I'm all set to OC with my build?
Yep, you're all set. You don't need a 750W for that setup to OC.