Also Beatport offers lots of tracks in 320 or lossless, so you can listen to them and compare (actually I'm not sure if you can preview in lossless?). I get all of my music in 320 kbps, but if there's an album/EP that I absolutely love, then I go for lossless. However, I basically only listen to EDM (electric dance music); if you listen to rock or something, then go for 320, because it wipes out recording errors. If you do download any lossless music, I recommend making an ALAC version from the right-click menu in iTunes so that you can play it on your iPod, and delete the original one to save space.
FYI, a 320 mp3 is usually around 14 megabytes in size, and flac is usually about 45. However instead of 320 kbps, it's up around one thousand.