AVG takes about 20mb RAM...its nothing. 512mb RAM and i play BF2 fine(lowest settings maybe, but still...). I prefer AVG, i did use avast a while ago, but i didn't compare it(wasn't into it back then).
Make sure you have a good fire-wall, IMO the best FREE firewall is ZoneAlarm. The pro version is amazing, but im not looking for the extra bits lol.
I use MS anti-spyware, SpyBot, Adaware and cool web-shredder for anti-spyware.
EDIT- 20mb RAM, not 4mb(thats low 4mb...lol). Looked at only 1 of 4 proccesses by AVG!