It depends on my system. Both my work MacBook Pro and personal MacBook Air have 8GB of RAM. There have been times where I've been 10GB in to swap, and the performance of the system makes me think I'm actively paging.

However, for most cases on my work/personal laptop the 8GB of RAM is plenty.

On my desktop PC, however, I have 12GB of RAM. I use this primarily for gaming, and I have definitely used 8+ GB on this box. The one time, admittedly, Windows Update was using about 9GB of RAM for no reason. But I have had games get up above 3GB in usage.

Also, are you sure your RAM is unavailable and not just used for disk cache or something?