What do I buy to upgrade my OS?

I'm currently running Windows XP, but I want to upgrade to Windows 7 Home Premium. I have all of the system requirements, but I don't know who to buy from or what to buy. Do I get the Upgrade Version, or do I just buy the regular OS? Which is a better dollar spent, a download or a disc (If I download, I'll have to run an ethernet cable through my downstairs, as the download will probably be too big.)? Please Help!