I think he is just wondering if he can have 3 of one kind and 1 of another, right? If you are, I don't think there should be any problems running 1 512 stick and 3 256 sticks.
Yeah.. I was asking because I have 3 256's now and 1 512... each of them will run together with the 512 fine but when I put them all in it won't start up.
Maybe it has something to do with the dual channel... one channel having more than the other. Did you try the 2 256 sticks in one channel and the 512 in the other channel? Try that and leave that 3rd stick out and let me know what the results are.