If you put the DDR333 memory in the first slot then both types of memory should default down to DDR333 speeds.
However if you put the DD400 in the first memory slot then the motherboard could try and run the DDR333 and DDR400 speeds.
This could lead to system instability or it might not even boot.
Depends on what you do.and what cpu you got
If you dont use any memory intensive program you wont need more than 512.
And running the ram at 333 might slowdown your system by a fair margin depending on your cpu...
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