Thinking of upgrading A-V receiver


I'm thinking about my next upgrade and am looking at a a few models up from my Marantz SR5600 for sometime in 2008. The ultimate goal of course is to improve the audio and to a lesser extent the video. Could someone explain to me if having THX Select2 and Dolby True HD will give me considerably improved audio? If I don't use an HD source (Blue Ray for example), will the Dolby True HD feature have any impact at all or must I have an HD source? Does anyone know more about THX Select2 for audio quality? My two favorite audio settings are Dolby PL II Music and Neo: 6 Music.

Is this Audyssey auto calibration just fluff or does it really work? Does it do more than just balance the dB's from all speakers? I know I'm asking for a ton of info. Many thanks.
I feel that the Audysey auto calibration is a very worthwhile feature. It's the common man's answer to expensive room treatment and professional audio calibration. In order to take full advantage of the significant sonic advantages that come with Dolby True HD and DTS-HD Master Audio, you have to have both the source player and the receiver with 1.3a HDMI to pass the uncompressed signal as "bitstream". Hope that helps.