@Hypnodad: Listening to the sound samples should give you a good idea of the DS-2's quality. Notice what a difference the recording environment makes. The best sound was in the acoustically dead closet, speaking directly into the mic at low sensitivity.
One challenge with voice recorders is that they pick up handling noise, which instantly reduces the professionalism of the recording. Using an external mic can help a lot as well as providing a fuller sound. Other readers have reported good results with Olympus's ME-15 mic.
Based on the specs on Olympus's site, the DS-30 seems to have marginally better frequency response than the DS-2; the higher price is likely due to its many interesting new features, few of which affect your goals. I'll try to get a model in for review.
For more on voice recorders, see http://www.oreillynet.com/digitalmedia/blog/2006/09/your_voice_recorder_questions.html
To layer music under your voice, you'll need a multitrack audio editor such as Audacity (free). See our tutorial (which includes lots of voice-recording tips) at http://digitalmedia.oreilly.com/2006/02/22/podcast-audio-editing-tips.html