If you’re a TechNet or MSDN subscriber, you can head on over to the links embedded at the end of this post to start downloading Windows 8 RTM and get your licence key.
If, however, you’re part of the general public i.e. without TechNet / MSDN access, you will have to wait till October 26th when Windows 8 is released to retailers. When it does, you will be able to upgrade to Windows 8 Pro from XP, Vista or 7 for the price of a $39.99 download. If you prefer a packaged, DVD version, that will cost $69.99. You can check out the features that are offered by different versions here.