Windows 7 Ultimate 64bit.
2xEVGA 480 GTX Superclocked
HP LP3065 30" LCD monitor (2560x1600 native)

ddraw and fullresolution=0x0 seems to work allright. Still, the best image I'm getting so far is output=direct3d, and fullresolution=original, with aspect=false.

Letterboxes the game left and right but it's almost fullscreen, and it looks good.

Oh well, as long as you're happy. =D

Hey, just got Dungeon Keeper myself and was surprised to find that it didn't even look as good as it did in the screenshots on GOG, but I've worked out what it is!

The game has two resolution modes (like Theme Hospital did) and for some reason it's on the lower one by default. Press Alt-R to switch, it looks great afterwards.

This explains why it didn't look as good as you remembered.

EDIT: Also, it appears this game is one of those annoying mixed aspect ratio ones.
The opening video needs aspect=true to be correct, but then the main game ends up stretched vertically. This is a rare occasion when I'm going to leave aspect=false. :P