When talking to my friends I would use headphones though.
This is key. Good.

Remember if you can hear it in the room, the mic can hear it too. Keep that in mind
I think if you got a small mixer with enough inputs and two sets of outputs with send levels per channel, you'd be fine.
Ordinarily each channel has gain and a fader which affect the main output. Lets call that
your output.
Often you'll see another level per channel. Something like aux send, control room, or subgroup 1+2.
It doesn't really matter which of these you have, as long as it's per channel.
That means you can pipe the computer audio (from line out to mixer), the ipod, the keyboard, and a microphone into the mixer.
Then you can adjust the faders to suit yourself and pop the headphones on.
Next you can adjust all the aux sends for your friends listening pleasure.
Those outputs would return to the computer line input, and on to skype or your streaming software.
Say, for example, your friends say your voice is too low, you'd turn up the aux send on the mic channel, right?
Make sense?
There are other guys around here who are more familiar with various mixers and their layouts so maybe they'll chime in.
Basically you want something with 8 inputs and two independent output mixes. OK?