Unable to Boot my PC when using more then 2gbs of RAM!

Hey, I'm in need of assistance. So i had 2x1gb ram sticks in my custom made pc. It ran fine and so I tried to add 2x4gb ram sticks to make for 6gbs of ram. my computer will not start up when using more then 2gbs of rams. I've now purchased Windows 7 and installed it (64x) with a new SSD and figured "hey now I have windows 7 64x and can utilize all of my ram (6gbs)" But to my surprise, same issue with not being able to boot up with more then 2gbs of ram.. I have used all possible combinations in the DIMM slots with the RAM sticks and still only 2gbs of ram will work with whatever combination(2x1gb ram sticks work, 1x2gb ram stick works (both new sticks goods)) They're DDR2 and all sticks have the same specs and are compatible with my motherboard (as all have worked to run my computer just not all at the same time (only 2gbs))
Please help in whatever way you can! Thanks!
http://www.newegg.ca/Product/Product...82E16820134636 - This is the new RAM stick (x2)
(will not even boot with 2 of these in the DIMM slots without my older 2x1gb sticks)
MOBO : XFX nforce 750a (maximum of 8gbs ram supported)
CPU : AMD Phenom quad-core 2.2 ghz
GPU - Radeon HD 7770 Ghz edition
SDD 128gbs - Windows 7 Home Premium OS
HDD 500gbs