I would keep the same kind of RAM. Mixing is asking for possible problems, not to mention running ECC & non-ECC (which I hope you don't want to do).
Click <A HREF="http://www.crucial.com/UK/store/listparts.asp?model=SUPER+P6DGU&x=18&y=17" target="_new">here</A> for a list of compatible memory chips from Crucial.
If you want better performance, lose the ECC RAM, and get non-ECC RAM.
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