For an overview of a DIY system, check out videoguys.com. Here's their most recent DIY:
DIY System
Also, a guy did a custom build on DVXUser. Here's that thread:
http://www.dvxuser.com/V6/showthread.php?p=1779682#post1779682
Also, as David said, throwing four SATA drives into a RAID will be plenty fast enough for SD, especially if you go with a RAID0 (no protection, but plenty fast). I have two 500GB SATA drives in a RAID0 here and it's fast enough to cut SD, DVCProHD, and all flavors of DNxHD, so four will suit you just fine.
External storage is nice if for no other reason than heat management. Having four drives stuffed in a computer case can add a lot of heat very quickly. As long as you have adequate cooling in the computer you should be fine, but it will be a lot hotter than if you ran your storage externally.
Good luck with the system build. Just remember to check their specs to the Avid recommended systems. Primarily, look out for the video card (go with a Quadro from the approved list) and go with Intel processors. AMD may work, but Intel definitely works.
If you build a solid system, post the specs here if you can. I'd like to see what you end up with.
Michael
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I'll be working late.
