Get more RAM.

I had a pretty similar situation, 5 boxing on one comp (6600 core2 duo, 2G, 8800gtx), getting 60fps(main, 15fps capped on slaves) everywhere, but would slideshow in areas with lots of people. Did two things, moved everything to one folder (helped lots more than I thought), and bought 4G of RAM.

Even though I'm using XP and only getting 3G of it, it fixed the problem. Pretty sure you are gated by the same thing given your description.