If you are interested in templates, I came across this page a few weeks back and bookmarked it as I thought it looked rather interesting. It a package with various templates for creating DVD graphics and sets. I think it is around $25. Probably not worth if it, if this is a one time thing though.
This is a great sire for downloading free DVD covers. You can download one to your desktop and bring it into PS as it will give the measurements for dividing your pages and spine. Could also be good to browse through for some inspiration etc.
Use Photoshop if you don't want any text on there, or don't mind a slight bit of furring on the text. If you're having text, work on bitmap images in PS and then bring it into another program (Quark, InDesign, Illustrator) to add the text.
These programs cost hundred though and since you're asking, I'm pretty sure you don't own any of them?