That is correct. I actually wrote MainSqueeze, so I'm more familiar with it than the other options, but I'd encourage you to try each, if you have the motivation. Squeezebox Server should automatically see iTunes and import its library. So, once that's set up and imported, all you'll need is the IP address of the computer running it. Once you've entered that into the configuration in MainSqueeze, you should be up and running.
One thing to be aware of... the Roku currently only supports MP3, WMA, and AAC audio, and some users have reported some strange behavior with AAC audio, so if your library is mostly made up of AAC, you might encounter some inconsistency in audio. As far as I'm aware, that's not specific to MainSqueeze, but I haven't had a chance to fully investigate it yet.
I've made a lot of headway but no sound yet. Maybe you can figure this out.
1) I had an old PowerPC Mac Mini that had Gentoo Linux on it. I got tired of maintaining that slow machine and put it on the shelf. I pulled it out this morning, reloaded OS X and imported my iTunes library from a USB drive I used for backup. Now I have about 800 hours of iTunes stuff on the Mac and it works fine in iTunes there. Being an iTunes server is the sole job of this machine going forward.
2) I got the Squeezebox Server installed on the Mac and verified it starts up automatically when the Mac is rebooted.
3) I added the Main Squeeze channel, configured it to point at the IP address of the Mac. After finally getting the IP address input correctly Main Squeeze sees the library listing lots of CDs. It also sees playlists on the Mac. (Only 4 as I haven't built any.)
OK - so at this point I mess around in the menu, find a CD, select it and hit play. A few seconds later I get a message:
An error occurred playing track. (Error details: An unexpected problem (but not server timeout or HTTP error) has been detected.)
I see someone talked about this error when they tried to play a vorbis file. In my case I've checked in Finder that the file types for everything in my playlist is *.m4a which I think is Apples name for mp3, but possibly I'm wrong about that.
Anyway, so far I'm pretty impressed at how far I got, but not getting sound sort of puts a damper on the fun! I hope you can give me some ideas.