I use the system to watch movies and to listen to music. I have a 50” 1080p HDTV, a HD DVR (HDMI), DVD player (composite/coaxial), Replaytv (composite/optical). I use my iPhone to listen to music (analog) and sometimes the radio.
First of all, it is BEYOND time to get a Blu-ray Disc player. You are missing out on the quality that even regular DVDs offer when you don't have something which upconverts, and are completely missing out on the quality of your display when you aren't renting/buying Blu-ray Discs.
The lack of HDMI ports, on screen gui and the lack of a quality interface to my music on my current receiver are the main reasons I’m considering upgrading.
The first reason is excellent, the second reason is good, the third reason is lousy. Horrendously lousy.
If you need a good interface for your music, then get a device which is designed to playback music. Your iPod is a pretty bad source, but AppleTV is a very good way to go if you want a solid GUI for accessing your music. The PS3 and XBOX are also pretty decent media servers. But, asking a receiver to do a 'good' job with handling music, is asking for it to do some multi-tasking beyond what they tend to be good at.
That said: Most CAN do it, they just typically suck at looking good, being ergonomic, and just plain making people irritated while you try to futz about with their mediocre controls.
Instead, focus on proper connectivity and overall quality.
Additional features that sparked my interest are Airplay, Pandora, Pass-through options, and upconversion and upscaling (whatever that is).
Upscaling and upconversion are pretty similar. Upconversion will take a source that is composite, and change it to HDMI. Upscaling will take a format which is 480i, and scale it to 1080p. So, when you start with 480i composite video, the receiver will spit out 1080p over HDMI. More accurately this is transcoding and scaling.
Denon AVT-2112CI
Yamaha RX-V671
Marantz SR5006
Denon AVR-2312CI
Yamaha RX-A810
Yamaha RX-V773WA
I'm not a big Marantz fan, but I like Denon and Yamaha a lot. You should go to a store which has them hooked up if you can to see what you like the best. Denon, I use regularly, and is very reliable with solid power output. Not very glitzy, but very reliable and high quality. Yamaha tends to be of very similar overall quality as Denon and is worth buying as well.
It is worth saying that for sound output you are basically making a parallel jump. None of these will add much, if anything, noticable in the way of sound improvements over your 2300. This is strictly going to add HDMI connectivity for you and transcoding/upconversion. You WILL get HD audio with Blu-ray Discs, which could improve sound some... But first you need that BD player.
Good luck!