After doing some experiments myself, I can hit enemies just off screen (1024x768) with a bow or magic using the control key to target.
Using the shift key to move the screen focus allows me to hit any enemy I can see with magic (I started using Lightning, then switched to Hell Spikes since it can not be resisted). However, enemies not shown on the mini map are not displayed when you move the focus over that area, so basically the range is a couple of screen heights/widths away from your character.
Using a bow, the range seemed shorter than that, maybe a screen and a half attacking horizontally and half a screen or so vertically (possibly in part due to nightfall just as I started checking it). I tested this in the wastelands, though, where there are lots of bushes and other obstructions. This may have contributed to the lower observed range compared to using magic. If you wish to narrow down the range for bows more precisely, feel free to repeat this experiment. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />
There is no benefit to increasing your sight range past the mini map size, except to clear more fog of war.
