ecc & non-ecc ram

I want to combine the two, 2x256mb non-ecc, 2x256mb ecc.

I heard that you can't do it.
But.
I just tried it now and I get a total of 672mb (even though 4x256 != 672.

How.
To.
Do?

edit: don't msg the bnc "foonrisoffline", its dead and i dont know the login :<