System not stable with 4GB of RAM

I have been working for weeks to get my system stable. After much testing, I have determined that the fundamental problem is 4GB of memory (2x2GB). With one stick of 2GB, I have no stability issues at all. With 2 sticks, the system is randomly unstable in dual channel mode, and mostly stable in single channel mode. Mostly stable means it does everything except resume from standby (sleep in Vista). With both DIMMs in single channel, I get a variety of BSODs after resuming from sleep mode. With one DIMM, everything is rock solid. I have Memtested the memory for many hours with no errors.
Is this a problem with the motherboard? An incompatibility with motherboard and memory? A Vista x64 problem? Is my PSU producing enough power for this system? I don't even know what to RMA!

Gigabyte GA-P45-DS3L BIOS F7
E7200 Core2Duo 2.53Ghz 1066Mhz FSB 3MB L2
SuperTalent 2x2G DDR2-800 PC6400 CL 5-5-5-15 (T800UX4GC5)
Seagate 500GB SATA2 7200 32mb HD
EVGA 8800GT 512MB Superclocked Edition
Ultra X-connect X2 550-watt PSU
Vista x64 SP1

The strange thing is that the system is totally stable and normal with EITHER stick in there. As long as I have one stick of 2gb, everything works as normal, including resume from standby. If I put both sticks in and bring the system to 4gb, I get the blue screens shortly after returning from standby. Something in my system doesn't like 4gb.
Yes, I memtested both sticks individually for hours...no errors.
Is it possible that this memory is just incompatible with my Gigabyte Mobo?