As long as both the TV and your system (Media PC, NAS, etc) have DLNA, it should work (for certain file formats of course).
I agree with Dan though, I would stick with an external box to handle this for now. The Boxee Box is actually really good at this now (after some initial hiccups at launch). It also has Vudu (soon to have Netflix) and others. I returned mine because I am not quite there yet with my NAS setup, but it was a pretty good little system.
You can even get an AppleTV, 2nd generation, and jailbreak it to run XBMC on it. XBMC can read network shares and play all sorts of media files.
The first thing you need to do is decide what media formats you will want to play then work from there. I rip all of my DVDs to MKV and Boxee works great with those.