i’ve successfully converted my collection (2500+ songs) of 256 kbps mp3’s to 192 kbps aac. it’s essentially mp4 if you look at the standard, except that it’s more advanced thus it has more benefits. it has improved compression which means that i get the same or better quality as my mp3 and it takes up less space. typically, a cd of mp3’s imported at 256 kbps was taking up about 120 mb. with aac i get the same high quality sound in only 80 mb. that means i can fit more songs on my ipod. aac also supports mulitchannel audio, higher resolution audio (rates above 96 khz), and improved decoding efficiency (i.e. your machine isn’t pegged when you importing). i was able to take my collection from 18 gb of space to 16 gb. of course while i was reimporting all of my audio i found a bunch of cd’s that i wanted to add in so now instead of having 2500 tracks i have 3000 tracks in the same space. not to shabby if i do say so myself.