Can android phones play DVD-format movies over WIFI? I have ripped my library of DVDs to my home server. I play them over WiFi from other PCs.

Can an Android phone with HDMI out do the same thing as a media center PC, playing these movies over the network on to a TV?
It sounds like you're trying to do something a bit like DLNA? This is the set of technologies/protocols that allow you to play media on one device on another device on a home network. It's designed for connecting things like media servers, media players, TVs, etc all together to play across the home network.

You might find the answers to the following questions useful:

Can I get DLNA on my Droid?

DLNA enabled media player?