Organizations often jump to solutions when thinking about making major changes to their website. This is frequently because they haven't first thought through the focus of their website. And the response to this shouldn't be saying that any particular discipline should be first. The point is that the first step, before diving into any particular aspect of planning, should be defining the focus of your site. This vision should clearly indicate what your goals are. Perhaps more important, the vision should clearly detail what is not going to be implemented (see Website Migration Handbook for more on vision).
The focus of your website changes should be implementable. RSG's recent "Beware user experience overreach in your website overhaul" and Gadgetopia's "Seduction by wireframe" discuss where user experience is explored before a reality check of whether implementation is possible. Defining an implementable vision does not mean coming up with a full implementation plan before defining your vision, but considering at a high level all the aspects that will have to come together to make the change. One way of looking at the implementability of your vision is to look at it as moving a weight across a distance, and should consider many perspectives including the toolset, integration points, how the tools were implemented, how the organization manages ongoing changes, metadata, content, templates, functionality, legal needs, relationships, schedule constraints, budget, and others. But the point is to look broadly at what you'll need to do to pull off your vision to see if it's possible. Some effective ways of getting to a more implementable vision is to prioritize your goals if you have a laundry list and also to list the risks to implementing your vision.
You want to aim squarely at the intersection between your desires and the realities you face, obviously prioritizing the objectives that further your business goals. Furthermore, you only want to get to a high level design (and for example wireframes) for those elements that will be implemented in the near term, leaving room for implementation that can react to changes in the future:
