Sorry for the confusion (again.:blusher: ) This time things look much worse. After I replaced the original GPU the computer worked great for about 20 hours- speedy too! Then, when I rebooted the next day it only powered up with a couple clicks of the optical drive and a short whirring of the HD and nothing more. The only lights are on the power indicator and the NUM CapLock.
I tried the startup diagnostic (FN+ power) and the lights and tones gave me the following sequence:
The three lock indicators flashing twice all together,
Caps only flashing for about two minutes,
All three flash once,
Trilling beeps as they each flash left to right in quick succession,
Three flat beeps,
Then continuous repeat of the trilling and flat beeps and flashes again.

This matches nothing I can find anywhere on the net.
Do you think I'm looking at a motherboard situation?