I have been doing a lot of research on this topic lately. While personally I am going to go with my own modified Mac Mini solution, I would recommend you look into the Linux based SOtM products offered by Sonore Audio. In addition, Small Green Computer offers Chris Connaker's (Computer Audiophile) Windows based CAPS servers that are fully assembled, as well as their own Vortex box products.
I have heard the Music Vault Diamond server used in a friends system and have no opinion on it other than it is a bit pricey. If you plan on just using the USB output then IMO there are better options.
I have heard the Music Vault Diamond server used in a friends system and have no opinion on it other than it is a bit pricey. If you plan on just using the USB output then IMO there are better options.