You might want to check out the Hercules DJ Console:
http://www.etcetera.co.uk/products/HER100.shtml

This functions as a USB soundcard (with two independent stereo outputs
- so you can have a different feed to headphones) but also includes a
control surface, WAV/WMA/MP3 DJing software, MIDI in/out, digital out,
effects, etc.

I think it costs a little bit more than the Indigo DJ (you were looking
at the DJ version weren't you? I think that's the only one with two
*independent* stereo outputs) but might be worth considering if the
other stuff is of use to you.