Need help selecting a mixer, and perhaps a sound card.

First, thanks for any help you may be able to give me!

I have a [Korg Triton Extreme 88](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%3A%2F%2Fwww.ebay.com%2Fitm%2FKorg-Triton-Extreme-88-key-keyboard-%2F130614485356&ei=M31rUaiXKq6o4AOS24DIBw&usg=AFQjCNHybFgazq_ydqTwV-VfnannCImSLg&sig2=9jdNBsHVUrvC6gFb8x7abw&bvm=bv.45175338,d.dmg), a Boss DR-880, and a computer. I'd like to be able to do the following things:

1. Play music on my computer (in something like iTunes), and play along with it live on my keyboard.

2. Play my keyboard (piano) along with a drum beat from the 880 that I have programmed into it.

3. Record music using Adobe Audition or similar software from both or either of these machines.

I'd like the output to be through stereo headphones. I think I need some sort of mixer--maybe a 5-channel one--to do this. Alternatively, should I use some sort of sound card that has multiple inputs and have my keyboard and drum machine input there, then listen through my computer's headphone jack?

Thanks for the help!