I can't set the resolution to 640x480. My monitor just says "Sync out of range".
That message normally comes out when a certain combination of resolution, refresh rate and other settings just can't work. In that case my monitor turns off and the digital display on the monitor prints the message I mentioned.
Not a big issue though, but it's kind of weird. I could run other OpenGL apps at 640x480 (and higher) without problems.
Performance-wise though, I'm not having problems with GZDoom. It's really fast, even on my medium-spec'd system. I'm lovin' it.

Graf, you rock my world haha