We're talking about the new models aren't we? Not 2nd-hand? The new ones give you USB3 which adds a lot of storage flexibility.
So are you considering the quad-core i7 "workstation", or the server version?
The server version comes with the mounting kit for the 2nd hard drive (which otherwise you can get from macsales.com).
As far as CPU power goes, I'd be surprised if the machine needed more than two i5 cores.
Up until last month one of my workstations was the previous generation of 2.5 GHz i5-dual and it was very respectable (browsing, compiling, running VMs, etc). Of course having more CPU power never goes astray, but it's a cost question. Media encoding would be the most power-hungry function I can imaging for a media centre of some sort, but apart from web browsing (meh) you haven't really specified what you'd be using it for. BTW that 2011-model i5 I had was beefed up to 16 GB RAM and internal SSD+HDD.
Geekbench scores:
- i7 2.7 GHz 2012 mini: 12816 (there's variation: I've tested this at 12979)
- i7 2.3 GHz 2012 mini: 11688
- My i3 thornbill HTPC: 8272
- i5 2.5 GHz 2012 mini: 7218
- i5 2.5 GHz 2011 mini: 7065
As you can see, there's a significant jump in overall power going from dual 2.5 GHz i5 to quad 2.3 GHz i7.
Thornbill does a great job, including encoding recorded EyeTV shows in the background. The machine it replaced was a 2007 Core2 Duo mini (GB: 2744) which clearly was
not coping.