Use iTunes. Its a lot better then Windows Media Player.

I dunno if your Sirius S50 supports AAC, but AAC gives better quality then MP3 with a smaller file size...

(AAC is the music layer from Mpeg 4 which underpins HD-DVD & Blu-ray)
(MP3 is the music/audio layer from Mpeg 2 which is the technology that underpins today's regular DVD's).

Regardless, iTunes can encode into both.

http://www.apple.com/itunes/download

If you need instructions on how to rip using iTunes, just repost and i'll walk you through it...