It depends on what encoder you use, what settings and what music. There are a lot of sounds where MP3 has a lot of difficulties with compressing them, and most of the time it's not the limitation of the encoder, but of the format itself. So if you have very good hearing, with some certain sounds you will never achieve CD quality using MP3. With some other, you could compress them a few times until loss of quality would be really noticable.
If you want good MP3s, rip your audio CDs with
Exact Audio Copy - the best and the safest ripper. If you have Plextor drive, you can use PlexTools instead, but for every other drive EAC is the best.
Then, compress your waves using LAME. Find LAME 3.90.2 or 3.92, 3.93 is not completely safe to use yet. Use "--alt-preset standard" command to achieve really high sound quality. This is a preset and it uses A LOT of code level tweaks, not available when using normal command line switches. This preset guarantees HIGH sound quality.
You can visit
Hydrogen Audio to learn a lot about psychoacoustic audio compression. You will find tons of information there, about other compression formats, too. MP3 is not the only and not the best format around, when speaking of sound quality.

But for most of the people it's good enough and widespread, so if you use "--alt-preset standard", you can be sure you're getting almost everything possible out of MP3 format without making 320 kbps files.