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I have about 50 gb of free space on my macbook and wanted to know how much would be good for windows vista.

It all depends on how many applications you intend to install in Windows. You could make it anywhere from about 5GB (if you don't intend to be installing anything) up.

Mine is 50GB. Then again I use my iMac's Boot Camp installation of Vista almost exclusively for gaming and need a lot of HD real estate.
 
the minimum space needed to install vista is 2.6 GB. so if you need to install a few games, itunes/quicktime, office, a few apps etc 20 GB should be adequate. i have a 2.6 GB vista partition and 3 games installed on an NTFS partition on my external drive. works well.
 
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