New build, constant freezing

I just recently built a new computer to upgrade for the next couple of years and have encountered a major problem: freezing. After the build for about 2 weeks I had no problems and then the freezing started. When the computer froze it caused the screen to freeze leaving whatever i was doing displayed on the monitor and a constant looping sound of the last note of whatever i was listening to. It would happen when I played video games such as Unreal Tournament 3 or Battlefield 2. I would try to watch movies and if the movie player was in full screen or a size close to taking up the screen it would freeze. If i ran multiple programs at once, i.e trillina, utorrent, Mozilla, a video game, lots of windows open, it is guaranteed to freeze. So I memtest'ed my RAM and the same freezing occurred almost instantly or shortly after starting the test. I looked at all my temperatures using the programs Speedfan and PC Probe II. Both have all temps in the green except for a SpeedFan "Core" temp of 51*C. With the temp gauges frozen on my screen after a routine freezing, none of the temps are out of the ordinary.

After countless freezing my original RAM dies and i can't even boot up and my sytem wouldn't post. I go to bestbuy and buy the cheapest RAM i could find: 1 GB DDR2 PC2-5300 667MHz. I replace the other 2 sticks i have with the cheapo version and the same freezing occurs after a successful post and boot up. I send my Corsair RAM back to the manufacturer and have it replaced with new RAM. The new RAM checks out with a 1 hour memtest. I open multiple programs and the freezing occurs again.

I swap my video card with my buddies Geforce 7900 GT. My Geforce 8800 GTS works like a champ with no problems in his system. With the 7900 in my system the freezing continues.

I have swapped my 650w powersupply with a 1000w and the freezing took longer to happen but it eventually happened. So i send my 650w back to the manufacturer and have it replaced. Still freezing.

If i try to memtest the system right now, the system freezes. I haven't replaced my motherboard or processor yet but i decided to try these forums in hopes one of you with much more experience with this than I could help me out. My system specs should be listed at the top. What do I do next?

(disclaimer: I am apologizing in advance if i posted this in the wrong spot, broke some forum rule about having a post thats too long, or was too noob and didn't post something vitally important about my system that was needed. Please bear with me. This is my first post.)
 
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