thanks again, have managed now to get them both working properly. It took me also 3 hours "try and error" and lots bluescreens. Unfortunately i cannot really say, how i did it exactly at the end, because I tried it many small different ways.
first, it was of course necessary to put the board/selector and bios on "dual" mode. after doing that, the cards stopped overheating like during my first try.
I put the middle display on 8800gts and the right/left ones on the 7800gtx, like in yr setup. But when I installed now first the 8800gts drivers, i was never been able to stretch something at this point, because I could set only "one display". The other two monitors were not reckognized, not even at 640x480.
When I installed now 7800gtx drivers, system crashed.
Than I tried installing first 8800gts driver again, but afterwards deactivating same, in order to install proper 7800gtx driver.
This worked....I had now left/right monitor working properly in dual mode, and the middle one was working as well, but without driver (640x480).
Tried to activate now the middle one, in order to get him working properly as well....system crashed. (became a little frustrated)
So, I did otherway around. First 7800gtx driver, which worked well again. Put them on dual mode and "primary use". Now the 8880gts driver for the middle one who was running without driver. I cant remember here, but think system crashed again, and magically after re-boot he reckognized the 8800gts as well and he showed me all three monitors in "display-menu.
But one more question....are you running all three monitors via DVI or VGA signal? (When I had them on DVI...the left/right display didnt get any signal at all. So I changed everyone to VGA.)