Hey all,
This is my second build and I'm fairly ok at this (or so I thought).
I haven't been able to solve my problem with start-ups and so here I am.

Email in 3 parts:
1. Problem
2. My system
3. What I've already tried.

1. Basically, when I start up my machine all fans go for a few seconds, Mobo green light goes on, everything's great... until it cuts out. And starts again. And cuts out.
Sometimes it will run for 5 minutes, other times for 5 seconds. I have no monitor on it, haven't touched BIOS or installed windows yet. I just want it to run smoothly at this point and that is not happening.
Possible clue: power LED on my case (it's an LED next to an image that looks like a can of soup) flashes red occasionally. That could be telling me that it's a power issue- but I ran the newegg.com power calculator and and my power supply is fine for what I'm running (please see below).

2. My machine:
ASUS M3N-HT deluxe mempipe motherboard
Hiper 880W PSU
Amd Phenom 9850 2.5ghz cpu
2x 2g memory
2x Geforce 8800 GPUs

Newegg tells me that it will need about 800-850 watts; my power supply is 880. So I'm cutting it pretty close and I might upgrade the power supply. But would it really cut out during start up if it were just a power issue?

I've heard RAM could be a problem, so I popped one out. Still had glitchy start up, although it ran for its longest yet (5 minutes until I stopped it) with only one DIMM in.

Please help. Anything that doesn't involve me buying new equipment to test what's broken is greatly appreciated.
I will buy new stuff if I have to... it's just expensive and I'd rather not. Y'all understand.

Thanks.

Welcome to PCHF.

If you haven't installed Windows yet that definitely make troubleshooting easier.

For now, recommend only installing one memory strip.

If you can, connect the Monitor to the motherboard and remove the video card.

1. Are all the fans running?
If necessary power down the PC and remove the chassis cover, then power up again to visually verify that the fans are working properly.
2. How does the startup look know?

I removed one memory card and both video cards. Same boot problem. Starts up for a second, reboots, reboots, reboots...

I didn't hook up a monitor- the system would not even have had time to spit out text I could read.

I guess this means it's not power? SInce the two GPUs account for half the power demand in the system

Try removing the AC power,
then remove the BIOS battery from the motherboard for 5 mins
then replace the battery and re-boot.
Go into the BIOS and save as default settings.
Make sure you disable the onboard card and then switch off, plug in your video card and re-boot.

Didn't work...
My motherboard seems to need a BIOS setting change to disable onboard video, and I can't keep it running long enough to get there.
I took the battery out then put it back in...
The only difference is that now when I power it on, it starts up for a second and then stops- no more feeble attempts to reboot.

Thanks for all the help so far...

I suspect you either have a bad motherboard and/or CPU.
If they are under warrenty, I'd trade them in.