The issue I have has happened with two different types of RAM. I started out with Crucial Ballistix BL12864AA804, 2x 1GB sticks, which are on the Gigabyte website as "recommended". It was on sale then with a rebate for like $30 for a set of 2, so I got it. Then I started having stability issues running it in DDR800. I didn't know as much as I do now so I didn't really check the voltage, and ordered two more sticks (they were cheap) and put them in. Well, same issue so I figured the problem was that the voltage supplied by the motherboard wasn't enough for the 2.2V that the modules require for DDR800 performance. If I upped the voltage manually, it would top out at 2.135 volts, and then suddenly my USB keyboard wouldn't work in the CMOS. so I decided to clock them down to 667 (actually like 650 due to CPU speed) and just live with it. That was fine for 2 years.
Well, I just decided to max out my system, and am moving up to win7 64-bit from XP 32 bit, so I figured I would get more ram. I picked up 4 2GB sticks of G.Skill Ram FS-6400CL5, and threw them in. As these modules run at 1.8V for DDR800 speed, I figured my problems were over.
I am currently running fine in windows, but I have had to move my keyboard back to the PS/2 port as it will not function in the CMOS or on the boot menu screen when it is attached to the USB port. I have USB Keyboard and Mouse support enabled in the CMOS, so I am puzzled. If i run the ram at DDR667 I have no issues, but if I put it up to DDR800, this always happens. So far the ram seems to be stable running at DDR800, and I haven't had any hangs or reboots, but I'd really like to know what the deal is with the USB and DDR 800.
I was using an Antec Smartpower 500, but that started developing issues where it had to "warm up" for a minute before it would actually be able to turn on. It was at that time 5 years old, so I figured the caps were starting to go or something. It was replaced a month ago with an Antec True Power 650. I have had the same problem with both PSUs.
The ballistix yes I have heard the reviews. But I was trying at that time to stick to Gigabytes recommended memory. It was stable at the higher voltage but the USB weirdness made me under clock it down to 667.
I have checked with CPUID and the G skill ram is running at 385MHZ at 1.8V with no issues, other than the fact that the USB keyboard won't work. USB mouse seems fine.
I don't know how it never occurred to me to plug the keyboard into a different port. I moved it down to the next group of 2, and now it works. I'll be darned. I just don't get why what speed I run my memory would have anything to do with that. the keyboard only draws 100mA from USB power.
Issue resolved I guess, though cause still unknown.
amd's cpus have built in memory controllers and have limitations depending on how much ram you use and how many sticks of ram you have installed. I have 4x2gig ocz fatality ddr2 1066 and a phenom ii 940. I have to run it at 800mhz because the cpu cant handle 1066 speeds when 8 gigs is installed.
odds are there isnt anything really wrong with your system, just a hardware limitation with your cpu.