You need to set your Northbridge voltage +.2v and the command rate to 2T. Since your memory controller is integrated on the CPU die, you would do well to increase the CPU voltage incrementally as well. You are also advised to drop your DRAM speed from 400Mhz to 333Mhz. As mentioned, it is never advised to mix and match DRAM, but since you already have them, it will not hurt to populate all four DRAM slots and see if you can gain stability. Do not increase your DRAM voltage greater than 2.8v.
You lose Dual Channel Mode but with the AMD solution that is not nearly as great a thing (~3% at best and usually no difference at all) as an Intel solution. I would Memtest the stick first though as some people will give product that is non working away rather than garbage it. Why? I have no idea
None of this means you will be certain to get the sticks to play nice together, but it is what I would try to see if I gained boot and stability.