Mac Pro Optimal Memory?

Hi, I have a Mac Pro, it's a 2009 model, Quad-Core with 12GB of RAM. I use it for gaming and video editing, but I've noticed that at times, especially when I'm running something like VMWare the free memory goes down to almost nothing (usually around 23MB...that's megabytes...yeah I know).

I also run StarCraft II and Diablo and I need to have Skype open to talk to friends who also play, and generally when that's happening I have about 1.8 GB available.

I do have other programs running, no AntiVirus software, but something called Clusters which compresses unused files automatically, time machine backups, and Growl, none of which I think take up too much memory..

So my question is, would upgrading to say 16-18GB solve any of these problems? Right now Diablo III lags the entire system when I'm Alt + Tabbed out of it, and so does the StarCraft 2 beta. D3 has an option to reduce background frame rate for this exact problem. Apparently the OS can take up to 96GB of RAM, but I feel that's overkill as I'm not doing any 3D modeling or using it as a render farm for video.

Anyway, what do you think? How much RAM is enough and would adding more solve this low memory and lagging problem?

Thanks.