Exporting Quick Times & Varying Sizes

My FCP 6.0.4 system is set up to digitize SDI NTSC video @ 720x486 10-bit Uncompressed via a Black Magic Design Deck Link Extreme card and all FCP settings from sequence presets playback are setup to match.

When I export my timeline via QT, I set it for all of the above, but it notes that I'm using "Medium Quality" (since when is 10-bit uncompressed 'Medium' quality?) and that my video is both 720x486 AND 629x478. In fact, until I set it to 720x486, QT tells me that 629x478 is the native compression!

I've had way too many crashes while exporting. Is any of this a tip off to anything I'm doing wrong, or should I talk myself into that new computer a few months ahead of schedule?