A small mixer like a Behringer MX802A would give you the benefit of being able to put 8 inputs into it and leave them hooked up that way.
For instance, you could hook your guitar up to a multieffects unit with a stereo output for channels 1 and 2, put a keyboard into #3, an external MIDI sound card into 4 & 5 and a turntable for doing samples into 6 and 7.

Then you take the 1/4" outputs and adapt them down to 1/8" to plug into the sound card on the computer. The mixer would provide phantom power for your mic and you'd have full control over the volume and EQ of all your instruments.

To monitor the sound, just put the stereo outputs into your stereo system RCA inputs. You use headphones for monitoring and then switch the speakers on for mixing.