System Troubles

So here goes:

My gaming build worked fine for approx 9 months then wouldn't send a signal to the monitor after an update. Checked everything over, reset the system and it worked fine.

2 months later, the same issue started with no signal and progressively got worse and wouldn't boot at all. Thought it may be a video card issue so tried to boot on the integrated graphics and it worked. Tested again with my second video card and same thing, no boot.

Got out the spare motherboard and tried everything again...same symptoms. Finally got a BIOS code which indicated a memory issue. Ok, then test with various sticks of ram. No go. Bought 2 new sticks of ram and it will work with one but not if I put in a second.

So everything works great on intergrated graphics and one stick of ram on either mobo.

CPU seems to be fine.

Would a defective power supply cause all this to the point where adding an additional stick of ram can cause several problems?

3 days of troubleshooting and I'm spent........

Specs:

Asus P8Z68-v Pro or Gigabyte Z77-UD3H
Nvidia GTX 460 2WIN
Corsair Vengeance DDR3 2 x 4G
OCZ SSD 120G
Seagate 500G
Corsair 750W PS

Thanks for any insight as help is definitely appreciated.