AFAIK: you can only get 480p, 720p & 1080i off an Xbox1.
Just how exactly, when my AMD x86-64 1.6Ghz Sempron and Sapphire x1600Pro HDMI can just barely crunch it as is!
Is another question, given the Xbox1 only has a PIIor P3II class Celeron running @ ca. 700Mhz w/64Mb RAM and a GeForce 2 or 3 class GPU in it.
It doesn't really scream PERFORMANCE to me in this HD World we are living in.
As for how to set this Res in (or on), the Xbox1.
You'd have to first have the Advanced HD Pack (Or something similar, I've seen these on oBay for around $5.00(USD) + twice that for shipping), without this 'Pack' you can not get HD outa-the-box. (i.e. you'd have to hard mod it),.
So assuming you have that part youd then have to have a NTSC (or a PAL Xbox that was switched over to NTSC via the Enigma NTSC / PAL Switcher), in order to use the NTSC 480P Resolution.
Otherwise I'd gather it would default to 576p (i.e. PAL), but I'm not sure if that would or could work though.
But, yes I think you going to need to try and recover your Xboxdash.
IIRC: it's very, very simple to do so long as you can still access some other Dash (Evox, Avalauch or XBMC),.
The trick is to find a Program called boXplorer (I never realized how old this Progie was ~It's from 2002~ and well it's still the #1 Program behind XBMC! for it's shear power!
Ok heres' the whole trick!
From (Insert Dash here), lauch boXplorer and insert a Game that has Xbox Live Content on it.
For Ex.
Knights of the Old Republic 1 or 2
Splinter Cell 1 or 2
Ninja Gaiden
etc.. etc...
In boXplorer hit the White Button and choose the D: Directory and find the Filename 'Dasupdate.xbe' and run it direct from boXplorer.
This should rebuild the then current Dashboard onto your Xbox.
I say then current, as the Games I have listed are some of my own.
As I'm sure you will notice they are all old too.
I do not have Live on my Xbox and don't ever have plans on doing so either, so I can not comment on weather there is a newer update then that or not.
But, this will at least restore your MSDashboard again.
I would know I've done this nummerus times myself on my Xbox...