I am actually just thinking of running ubuntu and having XBMC load at start up .
You basically have 2 choices:
1. Stick everything onto one box - Ubuntu + RAID + XBMC + SABnzbd, etc. + + +
Pros:
One piece of hardware.
Most cost effective.
Cons:
Be prepared to spend your waking hours keeping it all in sync.
Upgrading one bit without the others becomes a challenge. (I guess for some this is a pro and not a con)
Single point of failure.
2. Separate functionalities into different systems - NAS on one box, media player on another, applications on a 3rd, etc.
Pros:
Devices become 'appliances'.
You tend to install and forget.
Fiddling with one does not affect the others.
Better performance, reliability, etc.
Cons:
You need multiple hardware platforms.
More expensive.
After many years of running with different variations of the above, I've come up with the following architecture.
1. Dedicated NAS - unRAID
2. Dedicated Media players - XBMC (openelec / XBMCLive)
3. Virtualisation, i.e. App Server (ESXi) - on here I have a few Windows and Unix machines. This replaced around 5 to 10 of my original systems running dedicated applications. Each VM performs one specific function, so effectively they're dedicated as well.
The above architecture allows me to rapidly back up, move or restore a specific system. I'm done spending hours getting systems working.