It'll take a little long to describe my situation but here goes the story :

In January 2009 we bought (the OEM parts) two similar systems with just one difference. One of them had a Phenom X4 cpu and the other one (mine) a Phenom X3 cpu. At the beginning we had problems with both systems to power them on whilst having all of their ram slots being full. We decided to install the systems with just 2 slots populated and later try to install the rest of ram sticks. Both systems did succeed to support 3 sticks. We tried many different procedures to make the systems work with their fourth ram slots being populated. We waited for new bios updates and flashed the boards when they were available, we tried different ram sticks with different frequencies etc. One day while we were trying to install the fourth stick, the X4 machine did accept it. The other one did not. The most mind boggling thing was that after one of my trials the X3 system begun to not operate with the third slot populated. Our boards did have AMD 770 chipsets and we even tried to change the board of the X3 machine with another 770 chipset board. Now my questions are :

• Should we change the cpu ?
• What is causing the X3 system to not accept the fourth (or now the third) ram stick ?
• The manufacturers sites do claim that this boards do accept 4 ram sticks (but they only tested them with certain ram brands and models). What are the limitations for maximum ram configurations on motherboards ? Are there some "rules of thumb" except frequency, voltage, chip type considerations for which we did check our parts ?

Our boards are :

• Gigabyte GA-MA770-DS3
• Sapphire PC-AM2RX780 - PURE CrossFireX 770