If you`re going to use Vista and Leo ( iPC or any new distro using chameleon ) - it doesnt really matter, as far as i know. Chameleon will set your Vista boot automatically so you wont have to do anything - just install. So it`s almost the same in this case. I would go for 1 drive for 2 systems cause setting BIOS booting settings each time you want to switch systems can be really pain in the neck.

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