I will have to differ from opinions claiming all AV are light nowadays. On the last two years up to now i've used 4 antivirus, 6 months each. They were, in this order: Norton, Emsisoft, Kaspersky and ESET.
Norton was really light. Emsisoft was a bit heavier but it was also kind of light on my system. Kaspersky was a bit heavy and when tweaked for maximum protection it turned really heavy on the system. ESET, the one i'm using now, is the lightest i've tried. It's fully equipped with tweaking options and after configuring it for maximum protection is still the lightest.
Aforementioned, it's not all about RAM consuming. ESET consumes more RAM fully tweaked than Norton or Emsisoft did, but it's lighter. You'll have to try and test on your own system.