When the screensaver is active, I noticed less than a 1% increase in processor usage on a computer with 700mhz. It certainly doesn't use much and it takes only seconds to install.
I love my computer. My computer generates A LOT of heat, which increases the temperature of my room. When the house temperature is set to 65 degrees F, my room is actually 72 degrees F. It is very surprising that it generates so much heat. I've got several meters on it but it's normal for the type of processor and PSU I have.
The only screensaver that uses less resources than mine is the standard screensaver that just displays the "windows xp" logo. Quite natural, seeing as it's just one image and the image isn't moving across the screen.