DadBart
Basicly, there are 2 types of Dreams, dynamic and looped video.
Dynamic uses less CPU power because the dream is rendered by the video card not the CPU. Currently the only dynamic dreams are being relesed by Stardock as the software development kit to create them has not been released to the public. I believe all the dynamic dreams in the dream gallery are marked as such.
The other is looped video. The amount of CPU power that a Dream needs is dependant upon what type of video, wmv or mpg, is used to create them and also the resolution and bit rate of that video. If the dream was created by a WMV it will use more CPU than one created by a MPG. Also the higher the resolution and bitrate, the more CPU it needs.
I hope that helps explain the differences.
Lastly, if my understanding of the differences is wrong, I'm sure someone will correct me.