For what it's worth, the single-player Steam version of Dark Messiah seems to work fine in Crossover Games 10.1 on an iMac with 4GB RAM, 3.06 GHz processor, ATI Radeon HD 4670 graphics card with 256MB VRAM running OS X 10.6.8.
When using the highest possible video settings the game crashed, but on the lowest possible settings I ran through the entire prologue fine using a 1920x1080 resolution. I'm slowly adding the 'recommended' video settings with no problems so far.
I will try to post screenshots later.