Well the main thing I see is that you're your files are encoded at a very narrow bit range. See here: http://www.hydrogenaudio.org/forums/index.php?showtopic=18091
Basically you're encoding files with an overall bitrate of about 210kbps and a minimum of 192 kbps. If you want to have a fairly narrow range like that you could just use: --alt preset 210 which will give you an average bit rate of 210. You can change this number for a different average bit rate. The problem with this is it will probably not allow the bitrate to go up to 320kbps unless you set the number close to 320.
The other option you have is just to use --alt preset insane, which will record at a constant bitrate of 320kbps. It may be a waste of space, but you wouldn't have to worry about low bitrates.
I suggest you encode a few of your favorite songs, preferably ones that are pretty complex and might require a lot of high bitrate frames, using the --alt preset standard, --alt preset extreme, and --alt preset insane to see if you can tell any difference. If you can't tell a difference on complicated songs then you probably won't be able to tell a difference on simpler songs. The settings you use really depend on the individual song, the types of music you listen to and how good your hearing is.
I think I have pretty good hearing and the few times I've compared files preset standard files to preset extreme ones I can't really tell much difference. Most people use preset standard. I've encoded all mine at preset standard and the bitrate can vary a lot. For instance, Weezer-Butterfly is really simple acoustic song and only encoded at 134, whereas Weezer-Glorious Day is more complicated and encoded at 234.
Here's an explanation of some of the presets: http://www.hydrogenaudio.org/forums/index.php?showtopic=28124
If you really want to back up your CD's you should encode using a lossless format, which MP3 is not. I've never used one, but I know Windows Media Player 9 and 10 have a lossless format. Unfortunately it only compresses files to about 50-60% of their original size, so a long CD will end up over 400MB. Here's a forum you might want to check out as far as other lossless formats.
I hope some of this helps and doesn't just confuse you more.