Before you go and buy the Windows 8 upgrade or the DVD from retail store, it is best that you run the Windows 8 Upgrade Assistant from Microsoft. Currently, I am on Windows 7 Ultimate. I ran through the Windows Upgrade assistant and was surprise to see the possible issues that I would face.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips