For local streaming within your home there shouldn't be an issue. For streaming outside of your home would largely be dependent upon the speed of your connection. An average audio file will only need about 30-50KB/s of upload bandwidth. Video would require quite a bit more.
Also, keep in mind as far as video files go you will be relegated to DRM free .mov & .m4v files with the HP Mediasmart setup. You will also likely have issues setting it up to stream correctly via 3G. It's doable, but will require a more advanced setup.
An old computer running FreeNAS will handle streaming audio to computers and iPhone/iPad's locally with ease. However, to stream to devices (especially video in other formats/codecs) will require a setup that will transcode on the fly. The PS3MediaServer among others works well streaming media to multiple device formats. This can also be setup to stream out onto the internet and adjust the transcode for the bandwidth available at the time. You will also need to have an app that is capable of connecting to and navigating a UPnP Media Server.
I don't think the Mediasmart setup is going to be as simple of a solution as you seem to be looking for what you are wanting to do. For what you could do with the Mediasmart setup you could do for free (assuming you have an old PC laying around.)
