so "carry in" is quite literally in your case :)

I've been building pc's since '96.. granted that I make such a good job of them that there are years of a gap between builds :D ^^, heh.

In my experience THE most common problem when a pc goes tits up is RAM. Bad ram can do all manner or weird and wonderful things to a pc.. and coinsidentally it's the easiest and cheepest fix.

Try them one at a time, or go buy a stick of cheap stuff to see if it makes a diff.

To test the PSU, try unplugging extra things. I recommend the CD/DVD drive and the sound card if any, this should drop the power draw enough to see if the PSU is the issue. It should come on or stay on longer with less draw.

And if you need to calculate how much power you'll need for a specific configuration, you can use this tool : http://apc.canardpc.com/

I just got contacted by ASUS. It turns out that the integrated LAN was damaged by lightning through the ethernet. That's not covered by the warranty, but ASUS offered to replace the board with a new one for the equivalent of $40. So my beloved machine will live again :)
That's a great link. I like the way they give a breakdown of max/gaming/idle usage.