XFX 790i Ultra Sli Freezes

Probably all the BIOS tweaking is the culprit.

Have you tried auto or default settings?

Was the computer stable before and if so, what were you doing that right before the freezing happened?

Use Real Temp to check and record heat spikes while you game. If it crashes, you can back to the log and check the temps to se if they are the cause of the instability.

Crashing while watching youtube doesn't sound like a heat issue so you might have a combination of causes. Do you have the lastest drivers for your motherboard chipset?

Could just be hardware limitation and not heat or BIOS settings or softtware.
2 Gigs of RAM is sufficient for Win 7 when you do basic things but depending on games and viewing high definition and having multiple windows, multi-tasking, etc... you could be running out of RAM and it starts using your hard drive as memory. Do you hear the hard drive working overtime just as you feel the computer starting to get sluggish?
Try to find a matching stick or get a dual channel kit of RAM. Even if it doesn't solve the problem you'll get a nice performance boost from dual channel.

I used real temp and i had no temp peaks when my pc crashed, cpu was around 60C and gpu at 70C.

Okay now I'm confused. Is it super fast during gaming or does it super freeze during gaming?

When the screen will freezes, I can't do anything with my computer. My keyboard won't change between capslock, my mouse won't move, and I cannot do ANYTHING. I have to hit the power button on the computer case itself and reboot the entire computer.

Sounds like a bad video driver to me. I had this problem in past with my old 8800GTS, a fresh install of latest video drivers solved it for me. It's not the HDD or RAM I believe or so I can tell with whatever experience I have with such problems, because they usually end up with a BSOD, but we can't really rule them out either.

Fresh install of graphics driver and changing peripheral power chord of HDD may work.
Good Luck!

Like was mentioned a few times above, this type of problem is usually RAM related. What exact RAM do you have? It would really help if you could post the model # so we can look up the rated timings and voltage. The first thing to do when having freezing problems is to manually set the RAM speed, timings, and voltage to their rated specs in the BIOS then do an overnight run of Memtest86+ to test for RAM errors. Your motherboard owners manual will have instructions on how to set the RAM values.

My ram's voltage was set to 1.5v and not to 1.8 as its supposed to be, so i cranked it up and been using it without freezing for a day now =)
Thanks for all the help guys!