lorax - 3:40 am on Dec 23, 2012 (gmt 0)
It sounds to me like you're not quite familiar with how WordPress works. The answer is, someone needs to convert that pretty PSD file into a functioning theme that WordPress uses to display your content. You can have whatever you want - as long as you plan for it and build it into WP ahead of time.
If you want 2 or 3 or 5 columns, you can have them. Do you need to have another PSD? Not unless you want one to convey the visual design to whoever is going to build this for you. A good theme developer should be able to build out a 2 or 3 column layout pretty quickly so you can see what it migh look like - providing the design isn't overly fancy. If you want the ability to switch between the 2 or 3 or 5 column layouts, you just need to build the different templates (subset of the theme) and make sure each is designed/built the way you want them.
If I were going to have someone build this for me, then yes, I would want a PSD of each different template and page.