Coonah :
Hi celtic_warrior,
Firstly Windows 8.1 AMD drivers are working fine on my AMD machine running an R9 280 with Windows 10 Preview. Secondly, you won't have to pay for Windows 10 "later this year" as one other person commented, it will actually be free for a year from release date (29th July 2015)...
BUT ... then you will have to pay, and rumor has it that Windows 10 will be subscription based which no-one is happy about.
I am guessing that what will happen is the preview will become redundant and simply update to the full version automatically, therefore you probably won't be able to continue running the preview.
1. The Windows 10 RTM is only free if you are upgrading from a valid Windows 7/8.8.1.
If all you have is the current Win 10 TP, then you will have to buy it NLT Oct 1, 2015.
2. MS has categorically stated it will not be a subscription license after that initial year.
The "1 year" thing is if you want to upgrade from 7/8.1 for free. You must do this before July 29, 2016.
3. The Tech Preview is slated to turn off Oct 1. You will have to do/buy something before then.
Either 7, 8.1, or 10.