To be honest I don't know really if they are exactly the same, but it is the new trend and even scene groups have started using it more and more, especially for HD encodes (HDTV, BlueRay, HDDVD).

I have seen a file x264 with to audiotracks and multiple subtitle files inside the same ... mkv? container I think.

The quality is much better than XviD not only sizewise but also XviD seems to have slight problems with really dark scenes, x264 does not.

But for XviD to be as good as they can get you need several apps. I have seen guides (That I am not allowed to copy) that have many apps and scripts listed. And these things mentioned aren't easy to get hold of anyway. But the GordianKnot ripping pack would be first thing to get. Includes AutoGK, AVISynth and some other stuff.