Problems getting Windows to recognize 1 of 2 GB of ram

Hey guys, I've become dumbfounded with this problem so I figured my last hopes are asking you folks, here's the problem:

Windows doesn't recognize 1 of 2 gigs of ram, I have x4 512 sticks of DDR400 (PC3200) in the motherboard (Gigabyte K8N Ultra 9). When each pair is in the computer on their own, it's recognized, so the memory is definitely working. I have all of the newest motherboard drivers, running Windows XP Home SP2. CPU-Z shows that the computer has 2048mb, but only shows information for what's in slots #3 and #4, but My Computer only reads 1GB, and the boot screen only shows 1GB.

Both pairs of ram are in the OCZ Platinum line, one pair is OCZ Platinum Revision 2, other is OCZ Platinum, the voltages on both pairs are the same, the only difference is that one pair runs at 2-2-2-5 and the other 3-2-2-7. I've tried every combination of slot placement I could think of, but to no avail. The only way that the computer will boot is if I have a alike pair in #1 and #2, the other pair in #3 and #4.

Any ideas?

Probably incompatible RAM, bad RAM, IT IS NOT motherboard limited (checked the specs online). Try updating the BIOS there seems to be a new BIOS update released during November last year. (Version F9C Beta)