takes up 2% cpu power for me, but I have it constantly updating weather, time, memory available, and network traffic.
6MB of RAM....the only thing I dislike is when you put your mouse over the icons and drag it back and forth it really eats CPU cycles....but as soon as you stop it does so I guess it's not a big deal. You can disable the thing which expands the selected icon (makes it bigger and shows it's highlighted and that reduces the CPU cycles a lot