Firstly you could try the proprietary nvidia drivers and see if the system corrects itself.
I highly recommend to try this first, except of course if you got beef with non open source software running on your install.
Anyway, if you still want the nouvau drivers (hoping i spelled that right :P) or the nvidia drivers fail to fix your problem you can forcibly add your resolution (never ever mess around with the Xorg.conf files - seriously it can hang your system) with xrandr
cvt 1920 1080
this will tell you what option/arguments you should use for the next command
ie:cvt 1024 600
output:# 1024x600 59.85 Hz (CVT) hsync: 37.35 kHz; pclk: 49.00 MHz
Modeline "1024x600_60.00" 49.00 1024 1072 1168 1312 600 603 613 624 -hsync +vsync
so for the next command you should copy everything for the 'Modeline' and after:
xrandr --newmode "1024x600_60.00" 49.00 1024 1072 1168 1312 600 603 613 624 -hsync +vsync
then you must add the new resolution to your output (run xrandr to find your output name ie:VGA1)
xrandr --addmode VGA1 1024x600_60.00
finally to change to your newly added resolution run: xrandr --output VGA1 --mode 1024x600_60.00
PS: some or all the commands listed may require admin rights (super-cow) so before all that just run sudo su
to log as root
source (and more detail):https://wiki.archlinux.org/index.php/xrandr#Adding_undetected_resolutions