I'm pretty sure that Alan has commented here a few times that to his ears, even MP3s at 320 kbps are transparent (i.e. they sound the same as the source). I could be misquoting this, but if so, it's not intentional. Some folks certainly disagree that these files are transparent, and I truly believe they can tell the difference. Unfortunately for me though, I cannot, so I went with all 320kbps MP3s for ripping my CD collection. (
more about that here if you're interested, this was a related but broader discussion).
I'm completely in the minority I realize, but at least run an ABX test for yourself before committing one way or the other. MP3s are a great compromise of storage space, compatibility (more key for me than anything), and great sound. One good way to do an ABX test is to set up an iTunes (or any music player) playlist and include 3 songs. Two of the songs are the exact same song, ripped in either MP3 320kbps or in FLAC (or another lossless format). The third is the same song, but ripped in the opposite format. Then click shuffle on your playlist, hit play, and challenge yourself to pick the song that is different. When I did this, I failed every time (even luck would indicate I should have guessed right after a while, but I didn't!)
If your ears can pick up the difference, then definitely go with the big lossless files and be proud that you are indeed such a discerning listener. If not, MP3s might work alright for you too.