When I try to play downloaded HD videos under QuickTime, I get the same 'Buffer Overrun Detected' message. Lower-resolution videos do not create the same error, but are very, very choppy.
Here's something curious though: QT videos will play fine if they're streamed from, say, the Apple Trailers website, and downloaded videos will play perfectly if I copy them to an external hard drive and play them from there.
I'm running two 320GB drive in RAID 0. From browsing other forums it seems it could be a RAID issue:
http://www.nvnews.net/vbulletin/showthread.php?t=86131
http://www.geekzone.co.nz/hadders/2273
Perhaps QuickTime has an unsolved conflict with RAID arrays?