Unless you open CAD, 3dmax, crysis 2, video renderer like sony vegas and adobe premier or virtual machines at the same time 16 GB should be more then enough . However , Capacity is far more important than frequency and timings. Like for example , 2GB of 2400MHz would lose to 4GB of 1333MHz because neither photo shop nor high quality gaming can be done on 2GB of RAM.
I m currently running win 7 64-bit, and i have only ever seen my memory usage hit up to about 4-5 GB running games and other programs.
But you could use RAM Drive to assist your resource hungry Video Editing Applications. So with 16 GB you have 4 GB for your GPU's, 4 GB for your applications and 8 GB for a scratch drive or swap file drive.