It depends on what version you buy. If you have the 64 bit version of Vista, you will want to make sure yu install the 64 bit version of Windows 7. I believe that this time both the 32 and 64 bit versions will be on the same install disk, but I may be mistaken. If you are simply upgrading from Vista, then you shouldn’t lose any info and it should all stay in the same folders. If however, you are doing a complete clean install, you will need to back up your info on another drive, whether it be a second hard drive, a jump drive, or an external hard drive.
Win7 ships with two DVDs in the package: 32-bit and 64-bit.