As slhck has pointed out, skype is adaptive.
However, I did manage to find some rough numbers. Skype is said to handle 30fps 720p video.
Assuming it's encoding in something like H.264 then you'd be looking at something of the order of 4-5Mbps upload. (EDIT: apparently skype uses googles VP8 video codec.)
In any case, even on a lower resolution system you'll find the HD camera provides a much clearer picture to the other side.
See these links for more info:
How much bandwidth does skype need
New era in face to face
EDIT: Googled around about VP8. Apparently it's only advantage is that it's royalty free, unlike H.264. Other than that, VP8 is inferior in both picture quality and bitrate. :-(
Still, you probably wouldn't notice the difference. You'll still need approximately 5MBit/s at both ends to get a super fantastic image in real time.