Actually there are a few public channels that will allow you to stream from your PC: Chaneru, MainSqueeze (audio), Sunimi. And there are several private channels Roksbox, MyMedia, etc. Additionally the XDS has a usb port and there is currently a private USB channel (soon to be public) that allows you to play media from the usb drive.
The Roku plays mp3, aac, jpeg, and mp4.
On the audio side, it only supports MP3 and WMA. It doesn't support AAC, which is bizarre, because that's the required audio format for the HLS and MP4 streams it supports. It also supports GIF and PNG image files, and WMV9 SD and VC-1 HD video.