I guess I can mark this one as SOLVED, though I never did end up finding the real issue. After a long, long time and a lot of frustration, I threw up my hands and started over in the following manner:
-RMA'd motherboard again
-RMA'd GPU again
-New case (Antec 300 Illusion)
-New hard drive (Samsung Spinpoint F3 1TB)
Building it all again with the above parts got around whatever gremlins were giving me issues, and the computer is running fine except for one little startup hiccup (which I made a separate thread about).
I know it's not a great model for troubleshooting something, but at a certain point I felt like I needed to start over. Thanks to everyone that posted in this thread and helped.
