Microsoft's main ambition it to have win10 on billions of devices, its my opinion that not upgrading OEM's would be counter productive to achieve there goal ,,,many,many millions of devices are OEM's
I haven't seen anything about OEM's not qualifying. If MS excludes OEM's from upgrading to Windows 10 it will take them longer to reach their coal of 10 billion computers running Windows 10.
According to how much RAM you have, if it were me, I'd at least double it. Although my Windows 8.1 machine has 8 Gigs of RAM, I want to upgrade to Pro, so if it's possible, I'll also upgrade my RAM to double.
Edit: Whoops! I should learn to read closer . . . 16 GB RAM is probably enough.