There are two ways you can go:
OEM version - With this version, your Windows 7 is tied to one PC and cannot be used on any other PC. This includes if you have to swap out your motherboard. Also, you either buy a 32-bit or 64-bit version. You don't get both like the retail versions.
Upgrade version - with this version, you must already own a previous copy of Vista or XP that has a valid, activated key. You don't have to have it installed (although that requires some "hoops" you have to go through, but they're not that bad). Do a Google search on "clean install windows 7 upgrade" and it will tell you how to do this.
Here's one: http://www.winsupersite.com/win7/clean_install_upgrade_media.asp
BTW, as sneaky as this might seem, it's legal as long as you own a legitimate copy of XP or Vista (which means your key is genuine and has been activated).
That's it. The OEM is a "full" version and does NOT require that you have a previous copy of Windows. However, you're going to have to decide which do you want: 32-bit or 64-bit.
With the Upgrade version, you get both 32-bit and 64-bit BUT you have to already own a previous copy of Windows (XP or Vista).
Hopefully that helps!