Do I need a DAC?

I agree with Onemug. I wouldn't expect huge differences, but your amp and speakers should be revealing enough for you to hear a difference.

Your amp has a great feature that you may want to make use of. The amp and preamp sections of your NAD are connected by jumped rca connectors on the back. That means you can use it just as an amp or just as a preamp. If you were to get a DAC with a volume control, you can bypass the preamp section in the NAD. Doing it that way would probably make a much bigger difference. If you don't mind a little experimenting, you can try plugging your Airport Express directly into the amp section and use the volume control on your computer. You may be very surprised with the results.

"If you're not obsessed with a stereo that sounds good, sell your gear and buy a bose wave radio ;-)"

He didn't say he was obsessed with bad sound.

Actually, I think I can see why the OP was worded the way it was. I've read threads on this site were people would argue like its life and death over things like what kind of rca connector is best on a pair of IC's.

"I am mostly playing 265kbps or higher from iTunes, but not 24-bit and so."

Thats the your biggest problem. It really doesn't pay to upgrade your equipment to play lossy files. I would first worry about re ripping everything you have to a format's like FLAC, WAV, or Apple Loss-less. Once you do that, you will be able to get the most out of whatever equipment you choose to upgrade.

"Zd542: So in your opinion, a DAC isn't needed as long as I mostly play 256/320kbps files from iTunes?"

Maybe. I think a better way to put it is that you will get more out of a new dac if you are feeding it better quality music. Even playing mp3's through a new dac, though, will probably sound better just because of the fact that the analog portion of the unit will be of a much better quality.

In my first post, I recommended that you try bypassing the preamp section of your NAD. I would try it if you haven't already done so. Seeing how big the difference and what the difference is, may give you some insight on how to proceed.