When you have songs that are already in your library that have been ripped from the CD iTunes asks you if you want to replace those songs or not. So you can go ahead anyway, however there is a caveat I've discovered.
Sometimes (I think if the information in iTunes about a song has ever been changed) iTunes will not recognize it as already being there and will create duplicates. For 2 or 3 songs an album this doesn't seem like much but it adds up fast.
Just some "if I were you" kind of advice...adopt, adapt, and reject as needed:
iTunes will alert you if you're about to create a dupe and allow you the option of overwriting it or keeping it. In your Van Halen example, when clicking Import you should get a dialogue box saying something like "Some of these items are already in your iTunes music library. Do you want to delete them?" Say yes and you should be fine.
Even so you might find some duplicates get made accidentally. Sometimes, somebody has uploaded "updated" information to the CDDB (whenever I see this it's usually because somebody's been extremely nitpicky over something like "&" vs. "and") and you'll wind up with the same song twice, with slightly different titles. All you can do is seek and destroy the unneeded copies. Best way I know of is to do a quick check after ripping each album. Example: There are 14 tracks on such-and-such record, but in iTunes you see 15. Sort by title and you see that you have "Running With the Devil" and "Runnin' With the Devil." Zap one of them. (The older one if you've got a new preferred codec/bitrate: do cmd-i to see creation date and other info.)
"Show Duplicate Songs", under the Edit menu, can also help you weed out dupes, but it's based only on the song title, not duration or any other criterion, so its usefulness is limited.
Finally, if you're into AppleScript, iTunes is fully scriptable. I personally don't use it, but as my library grows the temptation/need to delve into it gets stronger...
I am going to rip about 400 CDs. How many GB do you think that would end up being?
Depends on a couple of things: the average length of the discs, and the bitrate you select. At 128 kbps, you get about 1 megabyte for 1 minute of music. (Vs 10 megs/minute on an uncompressed CD.) Let's say your discs are about 50 minutes each.
So, a 50-minute cd = 50 megs @ 128 or 100 megs @ 256
400 x 50 minutes = roughly 20 gigs at 128, 40 gigs at 256 (and so on).
Personally, I use AAC at 128, but some people find that noticeably lossy.
Do some tests. Seriously, I think you'd have to be a golden ear to tell the difference between 256 and 320. BTW, the Shuffle does have the option of making temporary low-bitrate copies for loading on the device while leaving your library untouched. (With the caveat that it takes forever because it transcodes on-the-fly.)
An easy way to delete duplicates after the fact is to display the duplicates Edit>Show Duplicate Songs then sort them by date Edit>View Options...>Date Added. Use the Date Added bar in iTunes to separate your duplicates from the ones just imported and the ones imported weeks/months ago. Then delete all the songs from the oldest date up to the day before you added music. Done.
Still, as said above, it is far better to catch the duplicates before you load them on to your machine.
And I agree that 320 is over kill. Yes, you want the best sound you can get, but remember that an iPod shuffle is not an audiophiles choice for music listening, nor are the headphones all that great, as well that the environment you will be listening to your music will attenuate your ability to hear properly. Can you really detect the delicate tonal differences of a musical piece using $20 headphones while sitting on an idling bus or walking down the street? In my opinion you would be unnecessarily reducing your ipods music capacity to receive negligible results. 160 bitrate is well worth considering.