Ich have a motherboard Asus M4A78-EM and changed the former Athlon 64 X2 2,8 Ghz CPU with a Phenom II X4 955. I have installed a new XFX HD7850 2 GB graphiccard, too. With the new CPU I can not use all my RAM. I used to have in Slot 1 and 2 each 1GB and in Slot 3 2GB. With this configuration booting is not possible. With only 2GB in Slot 1 and 1GB in Slot 3 the system works.
In the internet I found a lot about ramcontroller issues, higher voltage for the RAMs and broken slots. But I don't know how to find the true problem.
Really looks like your 955 has a bad DDR2 memorycontroler.
I ran into the same problem as you when I upgraded from a Phenom 9850 to a C2 955. with the 9850 I was running 2x2Gb 1066Mhz DDR2 corsair ram with no problems but when I switched over to the 955 I started getting BSOD, memory errors and sometimes it wouldn't even boot up. So i switch out the ram with this cheap 4x1Gb kit I had kicking around and I could only get 3 sticks in when I put the fourth one it wouldn't even boot.
Anyhow, long story short, the way I fix it was to switch to a AM3 mobo and I haven't had a problem with that cpu since.
PhenomII 1100T// Asus Sabertooth R2.0 // 2X HD5850 CF //8Gb of 1866Mhz ram, H50 cooling, HX850W psu all stuffed into a NZXT Phantom case.
Was a member of AMD Processer forum since 2008. Last post count 1818.
That is a good point, and the 780g chipset is quite dated at 5 years old. I would agree with Slayerx about upgrading to a new motherboard, especially since AM3+ boards are backwards compatible with AM3 CPUs. If you have the cash, I suggest the Asus Sabertooth 990FX Gen3 R2 and some good quality DDR3 2133. The board because it has features that make it reasonably futureproof, and DDR3 2133 because it is the same price as DDR3 1600.