What mobo do you have? I'd generally put two cards in xfire/sli in furthest slots possible and ALWAYS buy a board that optimises that. I have no real clue how it took them SO long to laypot pci-e slots a bit better. Even if on your mobo you'd be in 8x pci-e mobo spread them out more for better cooling.
As someone said, xfire does NOT work in windowed mode these days, used to, another MS condition of DX I believe. Not sure if Nvidia still do it, they seem to get more leeway from MS in bending the rules and still getting their drivers approved.
Kombuster and the likes will run windowed and not give you xfire/sli scaling. The logs are fairly long on afterburner, when you can again just load up a game then alt-tab out to check things are working.
As for the dying/dead computer, without more info its a pot shoot as to whats wrong. Have you tried to boot with only one gpu in, try any slot, boot without a gpu and see what beeps you might get. Sound like it could be the psu, I'd unplug it, take the cable out, hit the power button on the comp just to help discharge any capacitance and try it again in the morning or after a few hours.
Temps would almost certainly be down to two cards too close and top card getting very little airflow, but then again, kombuster/furmark will load a card to a fairly silly level temp wise, basically I wouldn't recommend running it, EVER.
