I'm running an i920 with 12GB Ram Raid 0 Intel SSDs and a GTX 580.

I recently had a stick of RAM die which reduced me to 8GB. I have windows 7 64. I have always disabled paging with this level of RAM and never had an issue with 5 clients, normally sitting around 8GB. I had forgotten about the disabled pagefile but after about an hour of play yesterday I received an unpleasant reminder as 4 out of 5 clients exploded and the 5th stalled.

I ordered some more RAM but ir was going to take a few days to arrive so I enabled the pagefile again. Even with the SSDs when I hit the wall a couple of hours later again I found some major lag again.

Moral of the story is buy plenty of RAM. I can't really contemplate how low your graphics settings and resolution must be to get 5 clients playable with only 4GB. This alone would probably be enough of an upgrade to solve a lot of your issues I reckon.