Using an external microphone will especially with low-pitch instruments give the other participant a richer and fuller sound.
We can recommend any of the following high-quality USB microphones. They are very easy to use as you simply plug them straight into your computer and are good to go. Please pay attention to that the Apogee is for Mac only.
- Røde USB Mic: http://www.rode.com/microphones/nt-usb
- Blue Yeti Mic: http://www.bluemic.com/yetipro/
- Audio Technica: http://www.audio-technica.com/cms/wired_mics/c75c5918ed57a8d0/
- Apogee MAC ONLY: http://www.apogeedigital.com/products/mic
All four mics are amazing value for money and come within the price range of 110-225 USD. You can also find good microphones around the USD 50 range from BlueMic.
As for the 4 microphones mentioned above: they have slightly different characteristics, but basically all do the same job very well.
The Yeti Pro has 4 different pre-sets of sound characteristics which can be useful.
The Apogee has audio input directly on the mic, which makes input levels very easy to control.