Hi there Savawa,
I have to admit to being a bit puzzled by your problem and it must be very frustrating for you. I know that you have run Memtest and that your memory modules seem to have passed but the kind of constant BSOD's that you are having would point to them being the problem. I am also a little concerned when you say that installing the 2nd RAM module makes things slower, it should speed them up and again points to a problem with the RAM. You say that the RAM is running at 1.5V and at 1066 at default settings but what are the full specs for your RAM? If the RAM is running under powered, this could by why your system keeps crashing. It could also be just a timing issue with the RAM.
A couple of other things to check:
Have you disabled the On-Board Graphics? Although there shouldn't be an issue between the On-Board graphics and the ATI 5770 there could be and disabling the On-Board graphics chip might help.
Do you have another add-on cards installed? If yes what are they?
What USB Devices do you have plugged in?
Are your Mouse and Keyboard new? (You would be surprised how often a dodgy Mouse or Keyboard can cause these kind of problems.)
Can you test your RAM in another PC or, better still, borrow a couple of sticks of RAM from a friend and see if they work in your system?