Hello Jozeph,
sevenal might be right:
http://www.tweakxp.com/article37161.aspx
I have a somewhat similar system running winxp 64bit, with 560W power supply (don't know how much of it is actually used, but my guess is you need over 400 to run SLI)
If that doesn't work go ahead and diagnoze whether the problem is HW related (MB / memory / HDD etc) or software (Windowz).
A normal ASUS A8N-SLI goes through the following boot steps:
1. SLI boot
2. IDE detection
3. RAID bios
4. Plug&Play detection
5. Device boot
From your description I assume all these steps are completed ok. To verify that, put a bootable CD in the driver and you should see "Hit any key to boot from CD..." if you don't see that message than something in your wiring is wrong.
In case it continues than there are some possible options:
A. Your HD boot sector is corrupt (this *can* be a virus, but can also be some bad HD)
B. Memory fault
C. Motherboard (or any card on the motherboard) shortcutting
To check this I recommend:
(1) Get a new HDD (can be an old small HDD you have lying around) install windows, see if the computer runs
(1.1) if it runs, I would try to low-level format the old HDD
(2) (option1 didn't work) Take out all extra MB cards (leave one 6600 on board) try to boot
(2.1) if it doesn't run you can try switching to the other 6600
(2.2) if it boots - put card one by one back in and try to boot (this will either find the culprit or point to a power supply overload)
(3) (option2 didn't work) you are in trouble

at that point I would check the actual MB and memory, but it probably mean you need to reinstall your MB
-- Disclaimer -- This is a free advice and as such I do not guarantee it will solve your problem or even put you in a better position than you started at, anything you do to your machine is at your own risk. Best of luck,
NR.