Well, after going through this thread, I have some questions and then some comments.
The fact you seemed to have a problem with the basic onboard video does not fare well for your system. What video output were you using and how was it connected and what was it connected to?
What drivers have you loaded from either the CD or off the ASUS site?
I had Windows update give me a video driver for my on board video and it crashed the system. Luckily it seemed to auto uninstall without input from me.
Some comments:
There is always a chance the motherboard is bad. If you can exchange it, I would think about it.
Remove the 4650 card until you get your system stable. I have noticed some problems being reported with that card and Windows 7. Even though it was not part of the original situation, it might be part of another.
The next time you reload the OS, you might try the 32 bit version to test.
Although I have no good reason for suggesting this, I would look into getting a slightly larger power supply, maybe 550.