I had the same exact issue with my backup board! I had 4 X 1Gb sticks of Patriot 5300 in it...the board has 4 slots & can handle up to 8gb of RAM, so I bought- for $30- a 4 X 2GB kit of Ram; 8GB total. This new ram was Crucial 6400 instead of 5300..both kits run @ 1,8V/Dimm. I could run up to 3 of the new 2GB sticks in the rig; but whenever I put in the 4th, no deal. I could even have one of the Patriot 1GB 5300 sticks in the remaining slot with the 3 Crucials occupying the other 3 slots, or leave the 4th slot empty...just couldn't get all 4 and have it run. So I called Crucial tech support; this knucklehead proceeds to put me on hold no less than 4 times to ask/ read or both what the hell to do. His amazing answer after 20 total minutes of waiting on hold is "Let's unplug a case fan" and see what happens." To which I replied: The Crucial sticks run @ 1.8 volts just like the Patriot sticks, I also have a 640 watt PSU, with no bells or whistles such as adapter cards or anything in the rig; plenty of elbow room, I would think. I then ask him if it could be the memory controller (on the Northbridge on these older boards) that's causing the issue...to which he replied...let me put you on.... At which point I just hung up. So, I'm currently running 7GB: 3X2GB sticks of Crucial 6400 & 1 stick of the Patriot 5300 1GB in the 4th. And to be sure, I tried different configurations & iterations of the 8 sticks to be sure they all worked properly individually...I think it's the mobo memory controller can't handle it.