Locating the Xbox's in a single place isn't really a viable option you're asking for even more problems.
Your original plan of an xbox in each guest room is the sensible option. That way you only need run one cable to each room. This would be Ethernet cable (cat5 or cat5e). Note that due to attenuation their is a physical limit of 100m or 370 something feet for a single piece of cable before it needs a repeater (not sure how big your Mansion is)
It would make sense to use 1000BaseT as your backbone given the cost differential is nothing these days.
A simple way to do this (given that you want to expand as you go) is run 2 cables from the the server room to the first floor. The 2nd cable is a hot spare. Connect this cable to a 1000BaseT Switch (not a hub this is important) with enough free ports for each room on that floor plus at least one other free port, so if you have 3 rooms you'd want a 5 port switch minimum.
The best way to cable rooms is to have an Ethernet port on the wall (just like a plug socket), the back of this port connects the room to the switch on that floor. Then you run a short cable from the wall socket to the xbox. The reason for this is that it protects your infrastructure from idiots. If they break the cable its easy to replace. If that is too expensive then run 2 cat5 cables to each room with one cable as a hot spare.
Now if you want to connect up another floor you just run 2 cables from the switch room on floor 1 to another switch on floor 2 (again 1 cable is a hot spare) and repeat the process.
The reason for a switch (not a hub) is that switches don't broadcast all traffic on all ports whereas hubs do (this is not the only reason but an important one).
Regarding your DVD's, as I understand it you have 1000+DVD's that you want to rip to a hard disk but keep things like subtitles etc.
When ripping DVD's your basic options are to either rip as
A) an iso (single file)
B) a .vob (typically multiple files, but can be a single file)
C) Convert to another format, eg. an .avi or .wmv (normally single file)
So far it sounds like you've been using A. My recommendation would be to do either A or B with CloneDVD. If I understand what you want correctly this will be you fastest/easiest option.
CloneDVD (about $30) does require a couple of clicks however, these can be done one after another (in seconds). You can choose a default compression level and importantly ripping preferences like rip 5.1 english audio and always rip english, french and german subtitles.
You set these by deselecting the things you don't normally want. It takes a couple of DVD's to get the preferences just right, but once sorted every DVD you rip will automatically use them. CloneDVD will rip to .iso or multiple .vob
For bandwidth you can use this calculator
http://www.hem.com/vod/calculators.htm to work out how much you'll need.
Choose one link from this list
http://3d2f.com/tags/lan/bandwidth/meter/ to check/analyse what bandwidth you already have.
Regarding your skin, IMHO (only) Aeon is the best for visibility at a distance (720p+) but can be slow (never used it on an xbox, only PC and MAC)