On good monitors or headphones you can hear the difference between 320kbps mp3 and lossless wav. Also, you can hear the difference between 16 bit and 24 bit, 44.1 khz, 48khz, and 96khz.
Seriously, on a good system, with the right program material (i.e. recordings that have actual dynamics), you can hear the differences. Listen to Pink Floyd's Dark Side Of The Moon lossless vs. mp3, it's always a great test.
If you're listening on typical PC speakers or iPod earbuds, you won't be able to discern these things, though, or at least to a far lesser degree.