One source I read is that the PC800 have to be in pairs but I'm not sure if that adding the one PC1066 to the board will allow the PC1066 to work at full potential as the PC800 is weaker then the PC1066 if it could work though..
Cost and other reasons is why intel is supporting the DDR even when the RDRAM is a far better ram choice for P4's at moment epecially if one has the P4 533FSB chips. I'm not sure if the PC1066 or PC1200 can work with the new P4's 800FSB I mentioned as I would rather have RDRAM as they work more effectivly with the P4 then the DDR do if I was building my own system.
I would be happy to have 512MB of PC800 in a system like yours.
I'm a bit distracted by a chat right now so ... heh
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