Depends how you install it.
If you do an upgrade to windows 8, you'd keep your files, but they'd all be on windows 8 and you'd lose 7...
If you do the custom install, you'd lose 7 and files would back up to a folder called 'windows.old' in the C: drive.
For you, best idea is to partition the disk and make a separate 'windows 8' disk partition on the main hard drive.
This way you'd have a fresh install of 8, and if you dont like it, format that partition and remount to the C: drive to get the extra storage space back.
Alternatively....if you have a spare hard drive lying around - just use that!