My theory all along has been that you'll need 1GB of RAM and 8GB internal storage to be able to run Windows 10 properly on a phone - as evidenced by Microsoft's rapid shift to 1GB/8GB configurations for even its budget releases in the last few months (e.g. Lumia 430, 535).
Begging the question of what will happen to all the other, older, budget devices like the 630 and 520 which have 512MB/8GB configurations. According to Joe:
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
'Features may vary' implies that the kernel and core services will be in place, plus the basics of the UI, but that some of the graphical bells and whistles, plus some advanced features, will be missing. Which is, again, very much as we expected.
The 8GB requirement, you may remember, was also predicted by us to be a hard limit and Microsoft had subsequently said that 'not every phone' will get the upgrade and that just the 'majority' would. Implying that the real budget devices with 4GB of storage simply wouldn't be able to cope with the upgrade or the size of the OS.
Also on Twitter, Joe confirmed that, again as expected, that only a subset of the device line up would get the Insider (preview) builds first:
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
These will be the Lumia 930 and 1520, almost certainly, with an outside chance of the 830 and 735. We have a sacrificial 930 here ready and waiting, Joe still says that Microsoft is on track to release the 'Insider' build in February, so not too long to wait.