no, the motherboard doesn't "post"...that's a bad sign i take it? ...come to think of it, i dont know if it ever beeped at me.
well, if it's never beeped at you when you turned it on and since it's a custom built computer, it's possible that whoever built the computer didn't hook up the case speaker correctly, or at all. that's also assuming that mb doesn't have a built-in speaker.
either way, it seems likely that it's not posting, since there's no bios and no beep. if it is, in fact, not posting, it pretty much has to be the mb or the power supply.
i'm not sure what the crazy dual output asus video card you're talking about is. i didn't think asus made video cards. if it's an SLI mb, it's gonna have an nvidia-based card. is it running 2 video cards? if the computer is running 2 video cards, it's possible the power supply got overtaxed if it's anything under 750W.