Deathspine,

I dont know if this will help at all but I too had issues getting my 280GTX to run in SLI. THey were both installed and windows did see them but my PSU(even a 1200W thermaltake PSU or OCZ 1000W) did not have enough A on its 12v rail. In the of the OCZ it was not insufficent amps that killed it but instead the cards spiked too many amps which cuased the PSU to go into safe mode and shut down. I am now using a Zalman with 6 seperate rails. If your PSU is not at least 1000W with muti 12v rails you will most likely find an issue there. Also, the GTX is fussy abotu which rials you install them to, try doing cross rail install, IE lets say you have Rail 1 and 2 each with a 8pin and 6pin connection, place rail 1's 8ping and rail 2's 6pin into one card and flip that for the other. As for not seeing the card, can you install your working card to the same slot and turn it on?(the card you know is working installed only). How about the new card in the slot that is working?(single card again). If both of these circumstances work your MB slot is good and your vid card is good, and we need to dig a bit deeper.

Mathieas