if you obtain 100% of your media from apple, front-row is the way to go, if you don't...
look at
plex, it's a OSX fork of XMBC, although it's undergoing a complete re-write currently so the next version will apparently not be based on the XMBC code.
as long as your follow certain naming conventions, it will automatically scrape your tv shows and movies, letting you browse them with full metadata. as well as keeping track of playback location (if you stop in the middle of an episode, and want to come back) and watched/unwatched status
It also has plug-ins to play stuff from different online sources, like netflix and hulu, although those aren't without bugs as they are community created and not necessarily from the content providers.
since it's been mentioned earlier, they are also working on eye-tv intigration, for access to broadcast programming. don't know current status, as i don't use this, could be fully functional now.
you can also install a small server app on the other macs on your network, and access their itunes and iphoto libraries from plex (in the next version, this feature will be greatly expanded)
One nice feature is access to other applications, you can select them, plex will quit and launch the other application, when you quit that, plex will re-launch. with remote buddy that i mentioned earlier, the change is seamless with your remote control. I use this to access
hulu desktop
so with a few programs it can do everything you want.
plex - media center app (free)
eyetv - DVR software & tuners
remote buddy - remote control software, greater access to both the apple remote and others.
the only real problem is playing media with apple's DRM, for that, you kinda have to use front row. but since you're currently using windows media center, I'm thinking this shouldn't be too much of an issue for you.