Hey PQ, mate of mine does a lot of DJing.  I'll ask him what he'd recommend and get back to you.

As per my mate:

Itunes desktop software actually has a system for this it allows you to select how many seconds at the end of one song mix in with the start of the last song, i'd recommend 7secs. This is very important as waiting for one song to end before starting the next will leave a slight delay between the songs which will be fine at the start of the night but disastrous at the end when you are trying to keep people up dancing. All you need after this is a jack to run from your headphones output to your amp input.

I bought a system called total computer dj in a box it comes with a physical mixing deck which helps as the night goes on. They are relatively easy to use but you'd want to make sure to put plenty of practice in and care in the set list if its for a wedding. Have the set lists ordered like, start of the night, womens dance, classic rock, end of the night, alternative, etc etc so you can easily go with the crowd.

So there you go, hope that helps.He has done a load of weddings but, of course, it always depends on what the couples want doesn't it?

why the hell would you need to play through iTunes? As long as all your tunes are MP3 format or the software can play back AAC files, then, no-problemo! To connect to a mixer should not be an issue if the input has a pre-amp, this is just a voltage issue, nothing to do with software, if there's no pre-amp on the PA input, running the output from your laptop through a 2 channel DJ mixer should work.

M-Audio do a number of audio interface products, but in fairness, probably overkill for a one-off event.

http://www.m-audio.com/

I don't DJ myself, but a lot of people us Abelton Live, which isn't DJ software as such, just a "live" sequencer, or there are more dedicated DJ programs like NI Traktor (about $100), but once again, this may be overkill.

many thanks folks, some top suggesting there. Binokular, relax with the itunes bit, i was just thinking of any pc based music player ;-)

cheers again!

Been doing it for years and have always preferred Native Instruments Traktor
(The current version is called Traktor Pro)

I recently bought the Numark OMNI Control
http://www.thomann.de/ie/numark_omni_control.htm

Couldn't recommend it highly enough, very solid AUDIO & MIDI Interface
(So you have full monitoring and midi control capabilities)
BTW the Numark OMNI also comes with the cutdown (but still very usefull) Traktor LE

With both the hardware and software combined you can mix any kind of set flawlessly