If you are experienced user just Windows Defender in Win 8 can protect you.
If you are inexperienced user, or reckless, clicking everything you see, sooner or later you'll be infected whatever security software you're using.
Specifically RAM usage doesn't determine how light is your AV, unless you have system that has very little work memory. It's very rare nowadays. System responsiveness, opening, writing the files (whatever the method or motive is), boot time, and cpu utilization on scanning by real time protection components determines how an AV impacts your system.
Yes, I can agree, if someone here knows something about Norton (and many other things), then
@n.nvt knows.
I am always in business to educate people how to protect themselves when using computer and browsing the internet. Not to blindly rely on an AV to protect them. My point is, both Avast and Norton are good programs, my vote will go to Norton because of it's SONAR, and that doesn't mean Avast is bad.
Thanks