The resolution of your screen tell you the number of pixels of a 2D plane. The fov tells you what portion of the view sphere is projected to that plane. Due to this projection, the view is compressed in the center and dilated at the boundary (which means that pixels in the center express wider angles than those at the boundary. If you think it should be the inverse, make a drawing, or think twice).
To put some numbers down, let's make an example only on one dimension: If you have an horizontal fov of 90 and play at 1680, a pixel will span (on average) ~0.05 degrees.
The mouse DPI determines how many ticks the mouse gets by moving it by one inch. By fixing a certain sensitivity (x inches per 360 degrees), you can compute the smallest angle you can rotate. I made a graph to show this:
With a 400DPI mouse, most people will have a precision below 0.2 degrees.
High res, low DPI, is this a problem?
No. With low DPI and high res, there is no influence of the resolution over the aiming capabilities.
Consider you moving your mouse. Your mouse input gets converted to the game internal representation of where your character is aiming, and than, based on it, a representation of the world is rendered. How accurate this representation is depends on the resolution, but such resolution will not influence the internal representation (you will always hit that spot, no matter how many pixels are used to represent that spot).
So the real question is if 400DPI is enough, or not, and I think that with all IE3.0 used among pros, this question should not be a concern.
What *might* be a problem is the inverse, high DPI and low resolution. In this scenario your mouse is capable of expressing rotations that on your screen look all the same, as it is simply too low res for representing them. This however is science fiction for quake, as nobody really cares about high DPI mice, and nobody plays on 200x150 screens.