what are the advantages of sa-cd over regular cds?

This might help.

The biggest difference is that SACDs can hold a high-res surround sound mix of the music.

The biggest marketing difference is that SACD is a higher resolution format that holds much more data, so it should be able to more accurately represent the music as it was originally recorded in the studio.

Here's a page about it: http://www.cdfreaks.com/article/95/8