First, the free versions are generally just fine for virus protection. The more expensive 'complete protection' suites contain additional protection against browser hacks, scanning of email, etc.
Second, ALL virus protection uses a lot of memory due to the shear number of virus signatures they must keep in memory.
Now, there are things you can do to prevent your virus protection from slowing down your game. The easiest is to tell the virus protection to NOT scan your game folder on access, only on write. That will prevent slowdowns when loading the games files.