I have had similar problems in the past with computers I have built, and I found that usually the easiest solutions is often the one that works.
I would pull out the video card (and sound card / nic if you have one) and reseat it in the slot to make sure it's all the way in, you shouldn't see much if any of the plated "teeth". Also do the same thing with your RAM and if you have the manual for your MB, make sure the ram sticks are in the recomended slots. Also if your video card has an exteral power connect, make sure that's plugged in correctly, you might even use a different power connector if one is available from your PSU.
It really sounds like something may not be seated correctly or be getting enough power.
Give it a shot if you haven't already and let me know if it works out for you.
**edit: Also, do you have 3 or 4 sticks of ram? I currently am assuming you have 3 since ram is manufactured as even numbered sizes (2G / 4G / 6G) That particular MB uses dual channel memory, so if you have 3 sticks, remove one of them. It's best to purchase the ram as a dual channel kit so they're matched.