UPDATE: 11/26/2015 - This guide is now a bit out of date. Look to the comments below for help!

If you are playing DS2 at the time this is posted, you probably have a pretty decent PC, and want to get the most out of DS2.

This guide will show you how to slightly modilfy the games files to let you run it at any resolution you want.

Go to this website, dedicated to DS2 mods.

http://www.dungeonsiege2.net/ds2fun/sysdat/

Then simply follow all the steps. Really, it's that easy.

After you follow all the steps on the website, be sure to go into the games Steam files in this directory (it may be slightly different):

C:\Program Files (x86)\Steam\SteamApps\common\Dungeon Siege 2

And double-click on "DS2VideoConfig.exe".

Then, set it to your new resolution setting and then close out.

This is something I initially glanced over when modding the game myself, you have to change some settings in the .ini file.

Simply go to C:\Users\YOURNAMEHERE\Documents\My Games\Dungeon Siege 2

Then open the "DungeonSiege2.ini" file. Change these lines:
width = xxxx
height = xxxx
To what ever your screen resolution is. For example, width =1920, and height = 1080. Also, change this line:
bpp = 32 (or 16, but 32 looks better.)

Then you should be able to play in in your screens true resolution!