I'm done with implementation: I have the following informations and comparisions:
# Objects: 4
# Triangles: 34,378
Render method: Raycasting
Render size: 640x480
Time without Octree: ca. 8 minutes
Time with Octree: 35 seconds (15s for loading and building, 20s for rendering).
And of course, the results look the same ^^. Thx a lot for your advices.