N900 is in theory DLNA capable, all you need is somebody coding an app for that.
There is a way, if you search the internet hard enough, to configure N900 as a media server which is pretty much DLNA but less plug n'play"ish".

In both cases you need a DLNA/WiFi enabled tv or some kind of WiFi gizmo connected to your tv. An easy start for configuring a media server is with your laptop and looking for tutorials on laptop->wireless->TV connections on the web.

I'm 100% positive that N900 can play 720p smoothly. The only trick here, is on how it's encoded. HD video can too be encoded decently, contrary to what most people/companies do.