Need some CLARIFICATION on GDR3 and Nokia's next flagship

So my phone situation... I'm on T-mobile (US) and the service in the city I work in is terrible. So I want to switch to AT&T as soon as I can.

When will the next Lumia flagship come out- this fall or early 2014? And I'm not interested in the EOS (or Nokia 909); I'm waiting for the Lumia 940 or whatever it will be called. If I have to wait until early 2014 then I'll just grab an S4 or something right now (maybe moto x?).

Secondly, I understand that the next Lumia flagship will be shipped with GDR3 - does that mean it will support 1080p and quad core screens? Or will we have to wait until 8.1 (early 2014) to get that.