Yes you can replaces your cams without replacing the heads. With that said, its not as easy as it was with the old pushrod motors. You can't just slide out an old cam and slide a new one in. You have to pull all pullies and accesories from the front end, remove valve covers, and remove timing cover to get to the cams. Since you've gone that far, you might as well pull the manifolds/headers and pull the heads to have them ported and polished. But that part is definatly not necessary, just an added bonus
Yes it is definitely worth it...its going to be expensive but you'll love it. I can't give you numbers because its all in what your objective is. Cams vary depending on your application. While your deciding on your cam setup, keep in mind what you want your future mod's to be. In other words, if you one day plan to go f/i set your cams up for that mod.