I'd suggest:
Ubuntu as base system
TwonkyMedia as the file server system for most stuff [supports SMB and Bonjour IIRC],
PS3 MediaServer for the PS3 [it's a simple java app, works brilliantly]
100Mb networking everywhere.
If you can't get wired networking, then ideally you want a transcoding solution. PS3 Media Server can do some decent downsampling of content to get it to fit on a slower wireless connnection where the raw file just won't work [I can't get raw 720p files to stream to my Xbox cleanly - the medium downsampling option works perfectly without too much loss of detail].
TVersity is also worth looking at, if you can get it to work properly - my install stopped indexing files properly. As I only stream to my netbook and the XBox, I just SMB [simple windows file sharing] for th enetbook, and PS3MS for the XBox.
As for replication solutions - well, with Linux you can use RSync. Any decent freeware backup solution should be able to do a daily copypasta of one drive to another.
Hope that helps.
In terms of a different machine for serving data, any low range quad core machine will have loadsa grunt for that. In terms of clients, AVOID ATOM LIKE THE PLAGUE and also avoid anything with an Intel 950, or in fact, any Intel chipset - only the most recent ones support HD resolutions out of the box with any degree of competence. Discrete Nvidia/ATI solutions are better.
I haven't experimented with remote power on as I tend to watch stuff on the Xbox in the evening - I go up to my room to kick off my shoes and stow my jacket, power up the machine, log in, start the services then go back downstairs...
Anyway, hope that helps. You'll want to look into things like TVersity, TwonkyMedia, FUPPES, and Bonjour for Windows, which I beleive works better on Macs than hooking up to SMB shares.