I did some searching for you.
Here's a micro-ATX motherboard with RAID5 included. An AMD, of course. AMD is the way to go now days.
Link
Since you are building an entirely new system anyways, this board costs you nothing "extra."
Other motherboars all with RAID 5 are
Here
If you need any help building your PC...I've built around 1000 for customers. So, I might be of a little help.
If I sounded rude at all I'm sorry, I'm just trying to help.
Edit: Also, I want you to realize that unless you want your HTPC doing the work of the receiver, your DVD playback software does not need any integrated decoders whatsoever. You specify the DVD software to use SPDIF out, and the receiver does the work with the digital signal. In my experience, PowerDVD is the best. Windows Media Player does a fine job as well, it has an option to use SPDIF out.
OH! Another Edit: Windows XP Media Center does a great job combining all of that onto an HTPC. Just thought I'd add that. DVD playback, recording, multi recording, playback, audio, everything.
