Hi...
Comodo firewall seems to work well, too, although I have not extensively tested or used it.
Your memory usage will
definately decrease with avast. just bear in mind that avast Home or Pro doesn't have all the built in components that NIS does, as Wyrmrider has mentioned. These functions will have to be filled by other 3rd party applications, the firewall being one of the main examples.
Avast has anti-rootkit protection, a web scanner, P2P and instant messenger scanners, Outlook and e-mail scanners along with the standard real time file scanner. Not bad for free.
The professional version also has a script blocker and other amenities not found in the free version.
Hope this helps.
Best Regards...