Now I'm back home in USA and tested the Amazon MP3 256kbps VBR files over AirTunes.
It's choppy. Boo.
My understanding of the AirTunes protocol is that the iTunes decodes mp3 files to WAV (or whatever raw data) and encodes them to Apple Lossless and sends it over the wireless network to AirTunes in an encrypted format. Fix me if I'm wrong.
So the fact that it's choppy with 256kbps MP3 but not with 128kbps MP3 indicates that it's a client problem? I'm running iTunes 126.96.36.199 on Windows XP and stream the music over AirTunes. The wireless connection is 802.11g directly connected to the Airport express.
I'll shut down bunch of apps on Windows to see if it fixes the problem, if iTunes needs more CPU to decode the higher bitrate files. Otherwise I'll copy the mp3 files to Mac and plays locally (without AirTunes).