So are you getting a new HTPC case?
What about a CPU cooler? Those OEM stock CPU coolers really suck (Intel & AMD). I think even a $15 Cooler Master Hyper CPU cooler does a much better job at cooling and is a lot quieter than those lousy OEM CPU coolers.
What about a 120mm chasis fan?
I have four 1TB Hitachi Deskstar HDDs; just like any HDD out there, those Deskstar HDDs will get hot unless you have a fan blowing at them.
I have the Radeon 5750, so I bet the 5770 will be even bigger. Is there room in the PC case for it?
Other than that, everything looks great.
Oh, and I would not do the RAID thing. What happens if something gets corrupted and it corrupts the ENTIRE 4 HDDs in the RAID?