Handy table of Windows 8 Upgrade Paths on Technet.com below, but also you should check out the Windows Upgrade Advisor  before you start that we recommend you run:

Before upgrading to Windows 8, we recommend that you run the Windows 8 Upgrade Assistant. It scans your current PC to see if it is ready for Windows 8 and then provides a compatibility report and optional steps for you to buy, download, and install Windows 8.”

The following table shows the settings, personal files, and applications that you can keep during an upgrade to Windows 8…

Installation paths to Windows 8 Keep Windows settings, personal files, and applications Keep Windows settings and personal files (data and system settings) Keep personal files only (data only) Keep nothing

Windows XP (Service Pack 3 or later)

No

No

Yes

Yes

Windows Vista®

No

No

Yes

Yes

Windows Vista with SP1

No

Yes

Yes

Yes

Cross-architecture installation (32-bit to 64-bit) Windows Vista with SP1 or later

No

No

No

No

Cross-language Windows Vista with SP1 or later

No

No

Yes

Yes

Windows 7

Yes

No

Yes

Yes

Cross-architecture installation (32-bit to 64-bit) Windows 7

No

No

No

No

Cross-language Windows 7

No

No

Yes

Yes

Windows 8

Yes

No

Yes

Yes

Cross-architecture installation (32-bit to 64-bit) Windows 8

No

No

No

No

Cross-language Windows 8

No

No

Yes

Yes

Get the full details on Windows 8 Upgrade Paths - Technet

Rob