Sandra Memory Bandwidth Test

I'm having a little problem with this program. I have 2x256MB RAM and 1x512MB RAM all @400mhz. When running the memory bandwidth test, it says it only tests 512MB of ram. Later, it tells me what I have in each slot, bank 0 is 512MB, bank 1 is 256MB, and bank 2 is 256MB. The 2x256MB are running in dual channel mode. My bandwidth ends up being 2000MB/1900MB, and it says my maximum memory bus bandwidth is 6400MB. Why are my actual numbers so different from the maximum and why does it only test 512MB?

I don't know why it only tests 512MB, maybe there are compatibility issues between the memory sticks. what are those sticks and what timings are they rated for?
Your maximum bandwidth in dual channel is 6.4GB/s that's right, but this value will never be reached in real life. I think sandra just doesn't make use of dual channel, as I get similar results with one stick and two sticks.
with an axp and nforce2 ram at 400DDR I get about 3050/2900 in sandra.
You may just compare your results with the built in values in sandra. try testing your 2*256 sticks (they are similar, aren't they?) and the 512MB stick seperated. you should at least get 2800/2700 in sandra with memory at 400DDR.

The above answer is wrong. Many boards with 3 memory slots only support 2 in Dual Channel mode, If you have a board with 4 slots then they 2 channels x2. That's why the 512 is excluded, if you were to swap 1 256 with the 512 then you'd see that the 512&256 are running together in dual channel. Your mobo manual should tell you which 2 slots are the ones capable of doing dual channel, but you've already figured that out.

Actually sandra by default is suppose to test up to 50% of the amount of memory only go in to options you'll see it there.