your iMac has extreme or normal airportspeeds?
because if that is extreme that will make a difference, and maybe you schould try something with the buffer in iTunes... don't know if it works, just a suggestion.
my G5 is extreme off course and my airtunes work fine... I used to have an Ti-Pb and I do remember it not being so great at wireless... but it worked (my iTunes library was on the network on my old B&W and it streamed to the iTunes on my Pb (before Airport Express that was a good alternative) and I remember I did set the buffer on highest and had no delay in playback...