I just picked up Insomnia simply because it is another Dark Tower tie-in. Got about halfway through it yesterday. A very good book! Suprise, suprise.
Another huge tie-in would be Salem's Lot.. because a character from that book plays an important role in the Wolves of Calla.
Actually, as was mentioned before, most of King's books are tied into the Dark Tower series in some way although I dont think you need to read any of them to enjoy the Dark Tower. They just might enhance the story if you had read the other books.
There are some things that have been introduced into the series that I dont like yet but with two more books coming out, who knows? I might, in the end.
Kind of funny, I've considered Stephen King as something of a hack for a long time. I read him a bit in my teenage years but became bored because it seemed like he used ideas from other authors and repeated himself way too much. The Dark Tower has changed my mind about him though. If he is remembered for writing anything good, this series will be it... at least for me.