On Windows XP, for purely business application use (not gaming or motion video), that Pentium 3Ghz should be enough horsepower for those two 1920x1080 screens. Vista/Win7 are more demanding, so that would be more borderline on the same machine.
Yes, you can add ports with a USB hub, no problem. The only thing to note is all those devices are sharing the same USB host controller & 480Mbps bus. So when you have a lot of USB activity from other devices, you’ll see the congestion first probably on your USB display.
Yep, using the VGA output is no problem - you only need one USB graphics adapter.
Thanks again for posting these questions. Hope that helps!
Bernie