I would like an HTPC that will be conected to my 16:9 widescreen HDTV.
 I want to be able to play any file off my upstairs computer.  I think
I'm planning on using the GeForce 6600 and MCE 2005.  If I have to,
I'm willing to build it myself.

Cable TV really is not a concern for me at this time.  My main desire
is to play any video, picture, or audio file, incliding Winamp
visulizations, on my entertainment system entirely in HD.

My questions are:

1) What min. network speed do I need to play video files (11g, 11a,
gigabit, wired)?
2) Does anybody sell such a system?  If not, what components (case,
cooling, cards, CPU, M/B, etc) should I put into this box.
3) I do have to use a HTPC right?  Or is there a digital media player
(like Roku) that will play any file in HD on my entertainment system?

This is a huge question there are so many possabilities. Network
speed. Forget it buy a video card with the proper pre-outs. If the
video plays fine on yoru computer than there is no need for upgrading.
It is all pretty much a persional choice. Yes there are media center
pc's, alot of places sale them.

Windows XP Media Center Edition 2005 is an operating system that
enables you to enjoy the best in home entertainment, personal
productivity, and creativity on your home PC in an easy, complete, and
connected way. With Media Center Edition 2005, you can store, share,
and enjoy all of your photos, all of your music, all of your home
video, and even recorded TV in one sleek and easy to use place.


Each new Media Center PC may include the following components to
enhance your entertainment experience:

? A Media Center compatible remote control that keeps all of the
entertainment on your computer within easy reach and complements the
keyboard and mouse.
 
? A remote infrared (IR) sensor that enables the remote control to
communicate with the computer and also controls your cable or
satellite set-top box.
 
? An advanced graphics card for displaying the best TV experience on
your computer monitor.
 
? A TV tuner1 that captures your television signal from a cable,
satellite, or antenna source.
 
? A hardware encoder that enables you to record TV shows from cable,
satellite, or antenna to your computer's hard disk.
 
? A TV output that allows you to display Media Center content on a
television connected to your computer.
 
? A digital audio output that allows you to integrate digital audio
from your computer into an existing home entertainment system.
 
http://www.microsoft.com/windowsxp/mediacenter/evaluation/hardware.mspx

Like I said in my question, I plan on using the 6600 which has native
composite output.  Also, like I stated in my question, I already plan
on using MCE 2005.  Also, like I stated, I want to play videos that
are stored on another pc, so I can't just forget network speed.  There
may be allot of media center pc's, but very few if any i think that do
what I want.