I've also found the AMD E-series APU to be a bit...lacking for XBMC use, especially when you try to play videos that aren't handled natively by the hardware decoder in the GPU. The CPU also chokes up badly on heavy skins on the E-series. For example, my OH's laptop (E-350, 8GB RAM, 1366x768 screen) chokes up using Aeon Nox skin. That skin runs perfectly on our two HTPC machines at 1080p - both HTPC are Intel based, one is a Celeron G530 and the other is an I3-3225, both using the Intel video. The CPU can choke up badly when doing other stuff in the background too - like moving files between RAID arrays, unpacking RAR files, etc. This isn't an issue with the i3, which is why I chose that CPU - with dual cores and hyperthreading at 3.3GHz, there's no issues with the i3 choking up.
what about the power consumption of the i3? if it is to be running 24/7 it can't eat 70W in idle. skin is not an issue the default one is just ok, i would mainly use the remote on my ipad.
don't plan to have any raid just a bunch of disks
One of the HTPC is also my NAS, hosting 3x2TB Seagate Green drives in RAID5, and 2x2TB WD Green in RAID1. There's a few other drives in there too, all in a Fractal Design Define Mini case, and the whole machine is cooled by 2x120mm fans that are barely moving. The PSU is an OCZ CoreXtreme 500W which is a VERY quiet unit. I just use the Intel heatsink, which is mostly stopped (Asus FanXpert keeps it off until needed). It runs MySQL for syncing XBMC libraries, Sickbeard, uTorrent, Couchpotato, SabNZBD+, and XBMC running ontop of Server 2012 which has been modified to look like Windows 8.
The HTPC/NAS is quieter than the electronic buzz emitted by my UPS!
So, to recap, your board choice is a good one if you;re running just a NAS, not so good in my opinion for doing NAS & HTPC duties together
if the i3 could suck only 18W as the e-450 i would go for the same setup.
i guess i will just buy a synology nas and use it with my raspberry pi, as it is quite hard to buy any decent parts in here