Thanks again for the additional suggestions Scat and blah12345678.
An update: Problem now resolved
I know you are all anxious to know what happened! LOL
Scat was right on. It was the CMOS battery CR2032. I took out the old one. Voltage still measured 3.1V or so. I bought a new one today. Voltage measured as 3.3V. Plugged in the new one. Installed the second EVGA 8400 GS card. Started up the system. Voila! Windows said "installing new driver". Screens blacked out... and then... 4 monitors got lights!
I had also reviewed the Xpower motherboard user manual. Apparently they have some slot population rules. For 1 PCIe X16 card, you put it in what they labeled as E2. (E1 is actually a PCIe X1 slot, which I don't use. It is so small, I missed counting it.) For 2 cards, E2 and E5. For 4 cards, E2-E4-E5-E6. For 2 years I have been using E2-E3-E4-E5. As it turned out, it didn't seem to matter. Tonight I had tested both configurations and everything seemed to work just fine either way. But I will stick with the manufacturer's specifications. Maybe it's for better heat dissipation, better power manager, whatever.
I think I had read somewhere about the motherboard scanning the hardware cards in those slots, and prep the information for Windows' plug-and-play. (But I couldn't find that webpage again.) That seems to make sense. CMOS power loss, configuration not savable with each boot. Windows doesn't see the additional cards, and thus not load the proper drivers.
Looking back... I should have changed out that suspected problem battery first before resolving to re-install Windows (even did that a couple of times). Time wasted for no gain. Just $1.99, would have saved me a whole afternoon and evening, not to mention a few hairs.
So... Thank you everyone trying to help me resolve this issue, especially Scat! Your kindness are much appreciated!
And blah12345678:
I don't have any AV software on this box. It is one of my 3 workhorse computers. I only use them for TradeStation plotting charts. I don't use them for anything else. They are to be on 24x7. I don't even want to reboot them unless I absolutely have to. My record is on this very box - it had a continuous up time of 121 days running TradeStation 9.1. I don't bother with applying Windows updates nor driver updates. Whatever worked when I first installed everything 2 years ago, I want to keep it that way. Power of the PSU is not a problem. I purposely oversized it. I have a 630W PS. I have only 4 dingy little 8400 GS cards and 1 disk drive inside. I don't have a UPS. Not a mission-critical box. And the electrical storm incident... it wasn't me.

Thank you for all your ideas.
Now I have 8 monitors back to work. And actually I got 2 more using 2 x USB-to-VGA adapters. For a total of 10 monitors on the same system. I had tried running a fifth 8400 GS card on the box 2 years ago. For whatever reason, that configuration never worked. Computer froze up, crazy screen displays, something of that nature. So drivng the 2 extra monitors via USB is fine for me. Unfortunately Windows 7 only handles a max of 10 and I don't have special software. I settled at 10.