Im currently specing something similar myself atm for my unRAID server.
I came across this little gem lastnight, Asus P5QL-CM.
Its matx, 6 x sata onboard, ICH10 so there nice and fast and its got some nice expansion options for adding extra controler cards if you want to get crazy with disks later on.
Only cavet is its Intel not AMD, but an e5200 would go very nice with it.
I realy wanted to find some Atom board that met my requirments but alas there just arnt any with more than 4 x sata onboard.
And because there mini-itx they have no expansion or very little posibilities
The power supply you have chosen is exacaly what I picked too. should handle a ton of drives and still have room to spare.
And speaking of drives, those Samsung F2's are pretty much the best bang per buck for storage atm
What OS are you planing to use, somthing windows or maybe somthing with a Linux flavor.
There are some very nice Linux file server options out there if you look around.