Hello,
Currently there is no way to achieve a bit perfect output from iTunes in the Vista environment. Some would argue though that the Vista audio engine is pretty good and question whether the difference between WASAPI (using say Foobar) and iTunes is audible. I believe they are audible - and afterwall, why should Vista resample my audio ? My own comparisons between XXHighEnd using WASAPI and iTunes did reveal a difference. XXHighEnd gave me an amazing soundstage set back just a little behind the speakers at the same time as making everything so easy to listen too. It's a similar story with MediaMonkey and ASIO - though maybe some of it is the placebo effect!
If certain conditions are met on XP then you can, I believe, achieve a bit perfect output from iTunes, depending on your audio card of course. A guy over a computeraudiophile did some tests and, with the soundcard unmapped, and windows volume control kept to full (or not loaded), system sounds turned off, he managed to confirm that the K-Mixer left the audio well alone. Obviously you need to make sure that your soundcard isn't one of the many that always resamples to 48khz, the XP standard.
I use MediaMonkey but the iPod Touch doesn't have as much control as it would using iTunes. I'm going to install XP on my Vista machine and give iTunes a try, comparing the output to Foobar etc.
Whilst bit perfect audio in iTunes is achieved on the Mac with no fiddling, I actually sold my Mac for other reasons. Mac users may smile at us Windows users when it comes to the above, but I wanted BluRay support and the iTunes HD downloads just do not cut it for HD playback. Upscaled standard def and 720 HD is not going to match full bit rate 1080p. Ever. That said, I believe that you could get around this by installing Vista on a Mac using Bootcamp - but watch out for those ridiculous Apple standards such as the mini displayport and check whether HDCP will be supported from Vista via this output. Think it will.
Neither OS is perfect. Mac OS, clean, easy to use, but you can only really do what Apple allows you to do, live within their contraints... Vista, messy, too many services running, needs configuring ... but hey, you're free to choose what you want.
Hopefully the above answers your question - though it was also a semi rant !
Remember also that using the Airport Express means your audio is limited in otherways. Before your audio get's to your DAC it's converted to ALAC (Apple lossless) on the fly. Whether this is again audible or not is entirely down to the individual. I for one simply don't want my audio touched, converted or resampled. A clean path straight to the external DAC is all I ask for ...
Whatever you end up doing though, by simply making sure your import your CD's uncompressed with error correction on means you're always starting off with a better file than any CD player. Error correction on the fly is simply not as good as error correction having been completed beforehand. This is the reason multiple magazines have agreed that the Naim HDX sounds better playing audio from it's hard drive than from the CD drive.
EDITED BY MODS for house rules violation.