You don't need a 7.1 system to listen to TrueHD or DTS MA. So far most of the audio tracks in both formats are only 5.1. You do need either your player or reciever to be able to decode the audio format. Many people are perfectly happy with a 5.1 setup, it's seems to be mainly of matter of personal preference. So to answer the question, yes, you can get HD audio formats with a 5.1 system.
I find TrueHD to be much cleaner in sound than the regular DD. This is especially apparent to me in the lower frequencies with gunshots and explosions. There is much more to the report and the sound is less "boomy," basically much better extension.
Jack