I said in my last post that I was going to try yanking out a stick of RAM. I had seen some mention of this in some message boards. I'm either really happy or really sad (undecided) to report that this seemed to address the issue (for the most part).
I am up to 3 screens, but down to 3GB RAM. Apparently, Vista 64-bit cannot manage 4GB RAM as well as 384MB video memory. I am still seeing the video driver "stop responding" but "restart successfully" every time I lock my screen and then log back in. But while logged in, the 3rd screen has been running smooth all day.
So, after all of this, it seems that this was never a problem with the video cards, but a problem with Windows Vista. I'll have to research it from that angle to see if I can find anything to address the limitation.