The terms "magic spells" and "magick spells" are used a little interchangably around here, namely because people are busy looking for both things on search engines and I like to keep everybody happy.
Personally, I lean more towards "magick" when I write. Ever wonder what the difference is (or if there is a difference at all)?
First of all, it's not a set-in-stone rule either way. Some witches spell magick with a k and feel that it's very proper, and some think it looks ridiculous. Whichever way you want to talk about your spells is up to you.
The idea is that "magic" is referring to stage magic and illusions, but "magick" is the real kind of magick that witchcraft spells are all about.
The spelling apparently came from Aleister Crowley, a well-known witch and occult magician from way back. He started using the term for these reasons, within the Thelemic religion and magickal practice that he founded. But it wasn't just to make a difference between real occult magick and stage magic.
The letter "k" is the 11th number in the alphabet, and Crowley was a big believer in the power of numerology. The additional letter not only creates a visually distinct word, but the "k" brings a great deal of extra power to the word through it's association with 11 (a Master number).
So when you're talking about spells, don't be surprised or confused when sometimes they are magic spells and other times they are magick spells. If you can't seem to remember the "k" in your own writing, don't worry about it. Your spells will still work.