No, I apologize for the confusing statement. I have tried four different graphics cards, but only one at a time. The four are 4550, 4650, 9800GT, and 6870. It seemed to make no difference which graphics card was installed. I have no interest in X-Fire or SLI.
My question was prompted by my belief that I have ruled out the RAM and the graphics card as responsible for the erratic behavior. I don't understand about the "8+2 Phase Power" design of the mobo, so I asked if there was something in this mobo design that required a special psu.
Strangely, I moved the pc from a corner table to a table in the middle of the adjacent wall, and the system is more stable. It now POSTS 50% of the time on power-up. It has not crashed since I moved it. Maybe some slight temperature difference? But the case is a full atx Lian-Li Lancool, with 3x120mm fans, a 95w heat pipe riser for the AMD 640 cpu, and the bottom-mount psu draws cooling air from below the case, providing a 4th 120mm fan (2 in, 2 out).