Computer with good specs running slow during games

Specs:
6Gb DDRII ram
AMD Phenom II x4 965 Processor 3.42Ghz
XFX Radeon HD 6950 2GB
500GB HDD (Forgot make/model)
550W PSU

If I forgot anything major, feel free to ask

So yesterday I was at my mates house and booted up Skyrim on his PC. His settings were at ultra ands he ran the game PERFECTLY. My specs are the same except he has the Radeon HD 6970 2GB. When I play it on my pc, however, I can run it only on low/medium with that same preformance. The same is true with plenty of other games (Battlefield 3, Mass effect 3, etc..). I have updated every driver I could, I have used 2 antiviruses which came up with nothing, I have stopped any background programs but still no luck. Any help? Thank you in advance :)

That one difference could be a very important one. Your PSU may not have enough amperage on its' +12 volt rail. Find out ow many amps your graphics card consumes verses how many your PSU supplies. If there isn't enough amperage supplied, you'll have to purchase a new graphics card and be careful on choosing one that needs less amps. Of course, you could always upgrade your PSU to one with pushing more amps or one with double or triple +12 volt rails.

(I am not knowledgeable when it comes to electricity, but here it goes)

I researched my GPU's power consumption and it requires a 500w+ PSU. Does that mean it actually uses 500w, or just wants that to be safe?
Also my processor says "TDP rating of 140W" and I'm assuming that means that it uses/requires 140watts.

Buying a new PSU wouldn't be a problem, I would just want to be sure that's the problem. Thanks :)

GPU power consumption values are determined at running the card in the worst possible conditions; like running them at 100% load at high temperatures. Refer to this link for better explanations than I could give: PC Power Supply Wattage

Same with the processor.