Its gonna be hard to find a decent AV that is light on the resources. All my clients here at work use McAfee and it seems to be pretty decent. It uses about 30MB of RAM in the background but rarely pegs the proc. If you tweak the AV's, you can usually get better performance but tweaking usually has negative implications.
My experience has been that McAfee is less of a hog than AVG or Norton. Not sure about other products though...
