I just installed a 400W Corsair PSU from Newegg, along with a Sapphire Radeon HD 5770, also from Newegg, on the same order. I'm having problems though.
Problem 1: Sometimes, upon powering up, the drives will spin up, and I can hear other noises, and the computer basically turns off, except the drives and fans don't stop. The light just turns off and then turns back on after about 2 seconds, and repeats the cycle, sometimes eventually turning on. I found that unplugging the 6-Pin from the GPU and plugging it back in will fix this and make it boot up.
Problem 2: Once powered up, if I try anything graphics intensive, like everything on Need for Speed Shift, there are white, pixel-sized specs almost everywhere, and the performance is low for what it should be.
Does this seem like a PSU weakness, or a Graphics memory problem like other articles suggest?