Ok, this Companion Cube is a white / grey bump mapped box with an animated Aperture logo and blue little lights. The animation is not perfect, I'm a beginner on Portal.
If you want (and if you know how to do it of course), you can change the fps number for the animation in the *.vmt file.
Actually I've made a previous animated cube earlier and thay match well together in my opinion.
You will loose some fps be cause of the animation on the cubes. It's not extraordinary, works well on my old P IV, but if you play with minimal graphic options already, this will be too much for your PC, if you have a lot of other mods, I think.
Here is a link for a video of this Companion Cube and his brother, my other animated Cube : click here