Do not mix stable and testing, they get too far apart. Much better to run straight testing for a desktop machine, or to mix testing and unstable. Personally I run straight testing until I know I want something from unstable. Unstable is usually pretty stable, but it is named Sid for a reason.
You can get a Testing install CD at http://cdimage.debian.org/cdimage/weekly-builds/
. Alternately, you can install Stable, change your sources.list, and immediately upgrade to testing.