My earlier SLI usage prompted me to use a Single 12v rail P/S, presently a Silverstone OP1000w w an 80A 12v rail, more than adequate for 2 8800GT in SLI, which I do have by the way.
Are you absolutely positive the 2 8800GT from XFX are identical models, I did get a set of BFG not at the same time that were manufactured in 2 different manufacturing facilities, and they didn't want to cooperate until I BIOS Flashed them both to the same BIOS.
NOTE: [BIOS Flashing a GPU voids your warranty, I knew that from the beginning and I'm not suggesting you do that, I'm just relating that there could be a difference somewhere in the 2 cards and if they're not identical your ASUS will have a problem with them.]
You can run GPU-Z on both the cards individually and get a read out on each one to compare both of their settings if they're not identical, contact XFX about it.
Its not impossible to acquire bad hardware, it happens unfortunately all the time, you could have 2 GPUs that work independently but not in SLI, or it could be a M/B issue, I'm assuming this is the first time you've attempted to run SLI on this M/B, Right? The M/B has never run SLI successfully with other GPUs has it?
You've already stated you had the SLI slot molex additional power plugged into the M/B thats very important, does the M/Bs CMOS setup have any SLI related settings that have to be enabled, I had an ASUS A8N32-SLI Deluxe, but I don't remember its setup parameters, but it seems there was some setting in the setup pertaining to SLI.
Just throwing out a few ideas here.