It would be prefered unless you can disable the onboard altogether, not many can because they dont build them to upgrade. That way you will have that used memory back to the system. Some are automatic with some of the newer motherboards for how much memory is allocated at one time. However most not all the time are you allowed to set it to zero with out it being disabled first. Being disabled altogether would help ensure you wouldn't run into irq conflicts tho, that is one thing to be cautios about if it doesn't include a secondary agp port.
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