768mb of pc2700 ram or 512mb of pc3200 ram?

I just upgraded my pc and plan to put my 512mb pc3200 stick into my aunt's computer. She only has a 256mb pc2700 stick.

I know that if I put both sticks in, the 3200 will run down at 2700 speeds. Would I be better off using both, or just the 3200?
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