Great you're now enjoying the iMac sound! However I do think you might for an even lower budget (US$ 25!) have tried improving your PC's software before buying new hardware. I had the same quality decrease vs. CD you got when playing ALAC files over AirPlay/ Airport Express via iTunes-for-Windows. But the difference between Airplay and CD disappears completely if I play the ALAC file with Foobar (free) and send it to AX via Airfoil ($25 license).
So to me it does not seem to be an ALAC, AirPlay, Airport Express or Windows-related problem. Rather, it's an iTunes-for-Windows problem. It's strange since ALAC and AirPlay were both developped by Apple, but anyway with my Windows system other software providers handle ALAC and AirPlay much better than Apple does!
I know this thread is one-year-old but I thought it was worth mentionning if other people get the same problem and consider buying new hardware...
