Depends how much the CPU offloads to the GPU card processor. I'd figure a few to substantial CPU use, depending on available memory to run graphics also. Full screen-saver I've seen it get quite jerky.

I run blank screensaver and occasionally the BOINC one if wanting to check out something. The BOINC screensaver can be set to go blank after x minutes. Saves cycles, and screen for those on CRT and power saving switched off.

While reflecting on the topic I on the side updated the screen saver FAQ in the Start Here forum and added a few glossies to show people the way what prefs to set and what order.... for instance idle crunchers must first let the core client start computing before the screen saver launches.

http://www.worldcommunitygrid.org/forums/wcg/viewthread?thread=4925#75214

Feel free to start a thread collecting some performance data for the graphics window and the screensaver. They are separate processes, so the taskmanager should log how much is taken on a given system of a fixed period of time to get to a % approximation.