The media centre version can only be obtained when you buy a media centre pc. I belive this is a standards issue. Media centre pc's need to meet certain quality standards and have certain component and hardware features then makes them ideal for running XP media edition OS.
There seem to be a number of OEM outlets who will sell Windows XP Media Center Edition 2005 now. It is still not available via retail. The software is not an upgrade to XP Home, but a full OS. If you have space on your machine I guess you could install it as a dual boot option.
You'll probably want to check out some of the "make your own Media Center PC" sites to determine setup quirks and hardware suggestions.