There is a superb (and the word is not strong enough) tweaking guide for Dark Messiah at TweakGuides.com. It provides some optimization and troubleshooting tips (but you might still need to contact Ubisoft for more severe issues) and points to other very detailed TweakGuides dealing with computer optimization, video adapter configuration, overclocking, etc. The articles are well written and easy to understand, and some of the technical information can be useful to Heroes V players as well. For Dark Messiah, I found the following suggestions particularly useful:

1. Disable HeadBobbing to avoid motion sickness, especially if you have a low frame rate. I agree with the author that the effect is excessive and unrealistic, and just plain annoying at times.
   
2. Disable Auto Save and save manually on a regular basis. While this is a useful feature in all games, it can also causes some stutter, and it happens quite often. Make a habit of hitting F6 after you made some progress in the game, especially if you feel that something big is about to happen.
   
3. If you own the retail version and want to update it via Steam, copy the .gcf from your Dark Messiah directory to your SteamApps directory, and relaunch Steam. That way, Steam will not download again existing files.
   

The official game manual is another reference that you can download from the official site. There you will also find a list of the fixes included in the upcoming 1.01 patch, which I will reprint here shortly.