You uninstall xbmc and install Plex.
Plex = transcodes video.
XBMC = no transcoding, plays all formats natively.
Seems like a no brainer to me. I'm sure Plex is probably pretty good but it seems like the above is a recipe for slowing down one's experience? Maybe I'm wrong.
To answer the OP's question, your next step is to look into downloading and installing addons for XBMC (e.g., video, audio and programs).
Not a particularly current, but reasonably representative list of addons you can get for XBMC to increase the content:
http://wiki.xbmc.org/index.php?title=List_of_working_plugins
NOTE: many of the addons may offer content that's legally questionable in your country, particularly if you live in the US. Be warned and use your own judgement and ethical calculus to decide whether you want to install these addons (e.g., Icefilms, Navi-X, and FastpassTV are repositories of a huge amount of copyrighted content).
However, many just make use of "scraper" software and aggregate content already freely available on the web - which is pretty cool. The Crackle Video addon, the Music Video Jukebox, PBS and Food Network Addons are a few favs of mine.
In short, there's lots of routes for using XBMC to drastically increase the functionality of your ATV2. It may take some tinkering, but it's fun and worth it.