Do I need a DAC?

Oscarrr, you don't need to be playing 24 bit files to be able to appreciate a better DAC. Most of my files are standard 16/44 and I find an external DAC necessary for the low cost sources that I use.

Oscarr, you have a great entry level system in the making. The express represents the weak link. A DAC would serve as an upgrade. The PS audio digital link III is often found used on this site for ~$350. Good DAC at that price, especially when avoiding the USB input. I play 44.1/16 bit files ripped from CDs.
The express outputs SPDIF?

"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.

Tonyangel: Okay, good to know :)

Mesch: Yes, the Airport express outputs SPDIF.

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

Oscarrr, DAC will give you cleaner sound. You might not care but noise embedded in music will screw up other things like for instance imaging. Noise on analog side is caused by the jitter on digital side and is detectable by lack of clarity only (since it is proportional to sound level). Your AE, according to Stereophile measurements, has decent/average jitter (258ps) on digital out but really bad jitter artifact on analog out, equivalent to 2.4ns - almost 10x worse. Read this: http://www.stereophile.com/digitalprocessors/505apple/index.html

Even with 258ps jitter you would be better off to place reclocker between AE and the DAC. AFAIK jitter becomes inaudible below 50ps (peak-peak) but it is a matter of individual hearing. Studio engineer or symphony orchestra musician would be more likely to detect lack of clarity.