If it's just purely a skype voice call, 30kbps up / down is the minimum, 100kbps is the recommended, more of course is better, note that this is just between you and one other person, more callers, more bandwidth.
Now, if you're talking about video calling, conferences and so on you're going to be seeing minimums of 128kbps all the way up to a recommended bandwidth of 8mbps for group video conferencing in HD.
For a complete list of bandwidth requirements, check out the Skype FAQ on it.
