Importing blog posts, photos, & website content from old to new site

I'm designing a new website using Wordpress to be hosted by inMotion. Although the design will be completely new (including using a different theme), I don't want to lose years of blog posts and carefully assembled photos/graphics. My current website (which is hosted elsewhere) will stay live until I complete the new design. So a few questions:
- How do I import my existing content? Can I save it as an .xml file? But then how do I bring that into my new website?
- Since I will be redesigning all the pages and rewriting much of the content (other than the blog posts), is there a downside to importing all this existing content? I'm assuming I can simply delete all the pages I don't want. But will there be strange artifacts that will cause problems?

Thanks! Michael