iMac: 120 GB
iPod: 20 GB
PC: 20 GB
I have a 3G 20 GB iPod and it's only about half full, so this is more than adequate for me.
My PC is used so sparingly now that a 20 GB HDD is more than enough. 😉
As for my Mac, I do have a ton of music, photos and movies, but having "only" 120 GB helps me to keep it nice and clean - I always make a point to have at least 20 GB free. I think if I had more space, I would just fill it up with more junk, and get lazy in doing cleaning. This way, when I watch a movie, I delete it. If I want to keep something, I burn it. Old music I never listen to? Delete it. I am a minimalist and although it's nice having every single episode of Doctor Who (180 GBs worth of episodes) I do not need all of them on my machine at the same time - that's what DVD-Rs are for. 😉
And plus, 120 GB is still a lot of space. 😱 😎