In DK1:

In the main game hold Alt and press R and you will find a considerble improvement in graphics!

Not sure why the game doesn't default to this but thought I would let everyone know.

Enjoy!

(I am not the finder - got from an old thread but thought it be good to share with the new players this weekend)

Slows the game down quite a bit though :(

I have not installed the GOG version yet, but I assume it uses DOSBox to start the DOS version of Dungeon Keeper. In that case all you have to do is increasing the CPU cycles in the DOSBox configuration. You can do this temporarily(?) while running the game by pressing CTRL-F12 repeatedly until it runs smoothly. Notice the text in the title bar when you're not playing on fullscreen, it mentions the current value. To decrease the CPU cycles, press CTRL-F11.