They are many AV's that are light now days.
Some may run lighter on some systems but on other may run a little heavier.
Since most systems now days have enough RAM, it is not the main concern, CPU cycles can drastically slow down a system even on a quad processor.
Light AV's include:
* Avira Personal (free), Premium & Security Suite (paid)
* Avast Home, Pro & Security Suite (paid)
* Eset Nod32 & Security Suite (paid)
* Norton AV & Security Suite (paid)
For Firewalls: All to a certain point will slow down performance especially the ones with HIPS features. Except for Windows default firewall and your router's firewall. The RAM doesn't make much difference, it is the pausing, stopping, allowing and checking CPU threads, tokens, modules and handles is what slows down systems. Some firewalls also can slow down browsing and connectivity.
It is trial and error to find a firewall that works well on individual systems.
As far as I'm concerned, there is really no such thing as a "light firewall". Once again just because it only uses very little RAM doesn't qualify to be light. Some firewalls can be big monsters and only use little RAM.
I have found that the firewalls included in the Security Suites listed above to be lighter than most stand-alone firewalls and firewalls with HIPS features.
ZoneAlarm Free runs light on some systems then runs heavier on others. ZoneAlarm Free is one of the last real stand-alone firewalls.
Online Armor Free and PC Tools Firewall can be customized to make it run light by disabling the HIPS features.
Comodo also can be customized when installing, if you only install the firewall and leave out the rest of the features.
Of coarse no HIPS features can equal less protection to zero day malware. If you know how to use HIPS correctly you will have better protection, if not then there is no reason to have HIPS because it will just slow down your system's performance.
Good luck.