Always happy to help whenever I am asked.
Much of Stephen King's work does tie into one another. Holding off on Insomnia until you are familiar with the Dark Tower mythos is good advice as there are specific references to the Dark Tower series. However, I don't think you should read all the Dark Tower books before reading Insomnia. I think you'd be fine reading it now; I can't remember any reference off hand that you wouldn't be familiar with. What Insomnia does, though, is introduce an antagonist that will play a major role in the later Dark Tower novels. There is also a supporting character in Insomnia that you might want to know about before reading DT VII.
Some of the connections are purely incidental and are not required to read in order. For example, the protagonist of The Dark Half is mentioned in Bag of Bones (another great read that you should get to), but you don't need to read the Dark Half to understand BoB in any way other than to say that you understood that reference.
I would recommend putting off "Hearts in Atlantis" until you're further into the Dark Tower series, though, otherwise some parts might be a bit confusing. Do you have King's collection entitled "Everything's Eventual?" It features a novella called "Little Sisters of Eluria," a prequel of sorts to "The Gunslinger."