I just have it on all the time in the background. Doesn't mean its scanning constantly. It does a complete scan once a week. And yes it is a good idea to have it on in the background at all times, especially when surfing the internet. It actively blocks any kind of incoming attack....Besides which it hardly takes up any resources.
Oh, well having it just up in the background is what most people do. It hardly takes up in memory that way anyway. I thought that you had it doing an active scan at all times. That would not be smart.
However, I still see no need for 8 Gig. You want to free up some memory, close a few programs. I see no reason to spend extra money when you can simply click the little red X.