There are a number of ways to go, and to add to the list, my method for my 4 rigs has been this:
1- Purchase OEM license for Windows 7 on ebay for $20. This is completely legit due to Microsoft OEM business model. The one caveat is that you have to do smart phone activation but it has worked flawlessly for me on 4 rigs.
http://www.ebay.com/itm/162148183120?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT2- After install of W7, upgrade to Windows 10 for free via Microsoft's "Assistive Technology" program.
https://www.microsoft.com/en-us/accessibility/windows10upgradeIn my searches, this was the cheapest and most legit way to get to W10, although certainly others may have found a better combination.
One thing that is not debatable though is that if you are planning to go with 6 or 7 GPU's, save your self a TON of hassle and go with Windows 10! I wasted many hours trying to get my first rig past the 4-5 GPU mark on W7 to no avail... once i upgraded to W10, I was EASILY able to get 6 working on the first try, with no extra steps (like running registry mods, etc) required.
