I was thinking in the BIOS itself - my ASUS 785G board allows for up to 0.3v undervolting of the CPU (e.g. that takes my Sempy 140 from 1.35v stock to 1.05v), and then you can also enable CnQ which drops it to 0.75v whilst idle. Your X2 might not go that far, but it should run with a bit less voltage and you can massively drop your power draw that way.
As far as cases go I'm as huge fan of the Antec Aria (sadly now discontinued) as an HTPC case: the new version is the
NSK-1380 but unfortunately it's *very* expensive (I paid a lot less than that for mine
) and it uses a custom PSU so you wouldn't be able to move your Seasonic over.
This Silverstone MILO ML03 looks pretty nice and takes standard ATX PSus, and I believe the
SUGO SG02B is always popular too; might be an option? Or you could always scratch build something, of course