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I just use OpenElec's version of XBMC. It resides on a 16GB SD card. I have a cheap MCE Remote and USB receiver that I use to interact with it.

It plays all my media (mostly 720p or 1080p mkv videos) which is stored on a NAS. I can also stream stuff from any of my Macs or iPhones/iPads to it via AirPlay.

It works very well for my needs.
 
If you got a mac mini and some external drives, let's say that runs you $800 bucks since you want at least 4tb storage.

Then, let's say the yearly cost of that is $100. And the Mac Pro was $200. So you're at $100 difference now.

To offset the cost of switching to the Mac Mini from the Pro, you have to run the Mini now for 8 years just to break even.

Math = win.

I would only "switch" from my Mac Pro if I was already looking for a new computer. But back on topic (since the OP asked about a Mac Pro 1,1 as a media server):

A Core 2 Duo Mac Mini (which is plenty for a media server... I had one before I got my Mac Pro) will cost you $100 on eBay. Toss in another $200 for 4TB of external storage (maybe less if you shop around) and you're at $300 up front. The cheapest Mac Pro that the OP linked to is $800, and only comes with 250GB of storage.

So not that it much matters the details of the power consumption at that point, but:

The Mac Mini (13W Idle/110W Max) uses less than half of the power of a Mac Pro 1,1 (171W Idle/250W Max. The Mac Mini + the hard drives is still going to be less than half of a Mac Pro.
 
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