You need a media server. Linksys makes(or made) a media center extender WMCE54AG. It lets you stream audio and video from your computer wirelessly.
D-link and I-O Data make units that aren't MCE devices but work with most file types.
They come with IR remotes so you can use your MX900 to control it. The D-link has a pretty nice on screen menu system.
The strange thing I've seen is that some of the companies that brought out MCE extenders have quit offering them.(HP,Dell)
You need to be comfortable working with your network settings, because port settings and firewalls have to be set just right.