The biggest thing is how much ram it will use. 32 bit systems are limited to between 3 and 4 gig of RAM that it will recognize. With Windows 7 and future growth in mind I would go with the 64 bit system. Basic machines now are 4-8 gig of RAM installed which will require the 64 bit system. Cheap machines will still have the 3 or 4 gig in them and may come with either 32 or 64.
Some old programs will only work with or at least work much better with a 32 bit system and that would be the only advantage to using a 32 bit.
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Windows 7 32 bit versus 64 bit
