You can also try Avast Home Edition. It's uses very less RAM. You can also juice up few MB RAM too!!
It has 7 detection engines -
Network Shield (LAN protector)
P2P Shield (not for torrents, for other p2p networks)
Standard Shield (plain old system protection)
Web Shield (web browsing scan)
Instant Messaging, Internet mail, Outlook/Exchange (microsoft products only)
Now if you don't have to enable all of these. For example, you can disable last 3 options if you don't use any of these. Also "Web Shield", if surfing is slow to you. P2P shield may be disabled if you're using "Torrents" for P2P.
So ultimately you will end up using 2 to 4 engines out of these. Thus reducing RAM usage.
I think Avast! is the one of the best detection ability among all of the AV. Also Home Edition is free for Life (free home license need to be renewed every year with zero cost). So you'll be using a legal product with mental peace.
Symantec’s latest offering is lighter than ever and an extremely effective security package. RAM footprint almost same as Kaspersky.
It seems Norton is back with bang.
details here: *www.pcauthority.com.au/Review/127995,norton-internet-security-2009-believe-the-hype.aspx