Which Version of Windows 8 do I need to buy for my Mac?

I plan on buying Windows 8 to give it a go on my MacBook Pro. I have had all the betas installed on my Mac in Bootcamp and while, it didnt turn me into a believer, I still will put it on my Mac if its not too much money.

The question is. Can I buy the upgrade version for my Mac? Or only the Full version?

I heard that MS said that Mac users would have to buy the system builder version of Win 8.

Anyone know?