Same thing here. The Nvidia install dropped me down to 640 also.
Re-booted & hit Esc, selecting Recovery Mode to get my choice of resolution
back.

Then on a 2nd PC which has only OB video, 8.4 left me with a max resolution
of only 800 x 600 on my 17" LCD.
However switching this PC over to a 19"CRT gave me the full range of res.
options.