I'm going to be building a system in an APEX MI-100 case. I already have the case and a 2.5" hard drive.
I'm planning on using a ZOTAC H55ITX-A-E motherboard with a Intel Core i5 661 CPU with an ATI Radeon 5670 and (most likely) a 120mm fan.
In SPCR's review of the APEX MI-008 case (internally identical to your MI-100), they note that the included PSU was actually pretty quiet! Is it possible to use it in your system? Or does it interfere with the CPU HSF (I assume it does).
Also, if this is a HTPC, do you need the Radeon 5670?
I actually have a Intel Atom D510MO and a hard drive in the case right now, acting as streaming media server, and the CPU is hitting 60F with just the power supply's fan (which is really quiet. I need to take the cover off of the case in order to hear it.). I'm just concerned about cooling a more power-hungry processor when most of the airflow path is blocked with the power supply.
This won't be a HTPC, this will be my general-purpose machine (I game a bit on the side, hence the 5670.)