Memory underclocked on second single channel

Hi all, I hope someone can make sense of this, because I'm stumped. I'm trying to isolate the cause of some random BSODs that I'm getting. First, the hardware.

MSI K9A2 Platinum, AMD 790FX/SB600
Corsair 2x2GB XMS2 DDR2-800 5-5-5-18 1.8v
Athlon X2 7750
MSI Radeon 4670, Catalyst 9.3
Thermaltake PurePower 500W

And here's the tests I've done so far:

1. Memtest, 2 sticks, 13 errors after 6.5 hours, all on Test #7 (various passes)
2. Memtest, stick A, slot 1, clean after 6 hours
3. Memtest, stick B, slot 1, clean after 6 hours

So I was going to proceed to test slot 2. This is where I get lost. When I boot with one stick in slot 2, the BIOS still reports "Adjusted DDR speed" as 800. However, Memtest, Everest, and HWMonitor all detect the memory as DDR-400 (base clock 200 MHz).

Is this something that should be expected due to some architectural limitation in the motherboard or the CPU's memory controller, or is something very wrong with this picture? I went ahead with testing in this state, which came out clean, but I consider it inconclusive since it's apparently running at half the clock that it's supposed to.

Yes I meant that it is running at the base clock, but that isn't correct. Everest is picking up the DRAM multiplier as 1:1, but it should be 1:2. It says 1:2 in BIOS, but I leave the settings on Auto so maybe it is doing something weird beyond the UI. Everest lists the memory "Real clock" as 200 MHz, "Effective clock" as 400 MHz, Bandwidth as 3200 MB/s, of course DDR2-800 is PC2-6400 so this is consistently showing DDR2-400 specs but the RAM is DDR2-800, and it shows up that way when I put it in the first slot.

Would this have anything to do with Ganged/Unganged settings?