Windows 10 will be Available for Phones with 512MB RAM, but Features will vary
Windows 10 for phones was unveiled last month by Microsoft and the first preview is expected this month. There was a huge doubt on whether Windows 10 will support devices with 512MB RAM as there are lots of budget phones in India. But now it has been confirmed by Joe Belfiore that all devices including 512MB RAM will be getting the Windows 10 update, but the only difference being some features may vary depending on the hardware.

Hey all: we ARE working on Win10 for 512MB, including 520. Our ambition is to make #Win10 available for these devices but features may vary
— joebelfiore (@joebelfiore) February 8, 2015
According to the tweet from Joe, 512MB RAM devices including Lumia 520 will be able to run Windows 10. He also went on to say that release of preview for these devices will vary, so this means it will be a phased rollout.
Timing for various phones may be variable as well. When we first launch insider program, it'll be a subset of phones with more coming latr
— joebelfiore (@joebelfiore) February 8, 2015
Further he mentions that the preview build is under testing and company is on track to release it in February as promised.
Also, we're on track for Insider build coming "in Feb" as Terry said at our 1/21 event. We need to complete internal testing 1st, hang in!
— joebelfiore (@joebelfiore) February 8, 2015
