There is a list of six free firewalls in this topic
. You'll need to scroll down a ways to find them.
I personally use Kerio Personal Firewall - the paid version because I wanted to have the privacy protection. According to Ultimate TroubleShooter, there are three running tasks related to this firewall and my configuration of it. At the moment they are using: 1.7, 1.7, and 4.5 MB of RAM for a total of 7.9 MB RAM. I have 256 MB of RAM total on my computer.
Why don't you try them out one by each and see how you like them, how it interacts with the other software on your system and how light they are? Sometimes its more a matter of personal preference and how your computer is set up. All 6 are good firewalls.