itunes quality settings

Increasing the quality of iTunes … How do I do ?!?!?
General Question: How can I make iTunes play my songs with the best quality? Specific Question: In the configuration, has a place where you can adjust the quality of imported songs. For further kbps quality means better or worse? Does the change of "transmission buffer size" do anything. Thank you!
Songs that its imports are always the same quality as they were. If the 320 before, it will be 320 in iTunes. However, when purchased, are in 128kbs. Kbs more, higher quality 320 is the best and the lowest should be viewed is 98
Match the volume and bitrate in iTunes – mp3 – AAC