Aie aie aie. I also had a Skype interview recently. Have NO idea what kind of impression I left- it was, to put it bluntly, a weird experience. I'm not a video chatter by nature, so staying natural was my biggest concern. As far as clothes: I wore a blouse and jeans. Professional, but on my grad-school-budget level. My interviewers really wanted to know why I wanted to go to that particular school, what kinds of research I was interested in, and WHY I was interested in that research. The last question really got me, because... well, I just like it. So: make sure you have answers to the "why" questions.
Also, if you can set up your computer in a place that gets full natural light, do it. The natural light looked so much better on screen than harsh university lights