
Windows 7 Pro or Enterprise owners can update to Windows 8 Enterprise with system settings, personal files, and applications intact.
If you are running Vista without SP1 will only have personal files preserved. Vista SP1 users will have personal files and system settings transferred.
XP owners with SP3 can upgrade directly to Windows 8, but only personal files will be saved. XP SP1 and XP SP2 users, well, sorry.
In regards to cross-architecture upgrades, 32bit can only upgrade to 32bit, and the same for 64bit.