Just a followup: turns out the tech was *right!* I've been searching
for more info on this problem, and it appears that at least one of the
ATI-based motherboards from MSI isn't compatible with nVidia graphics
cards. I found many threads on their discussion board about it; one's
at 'http://forum.msi.com.tw/index.php?topic=88430.0'.

Possible workarounds are to use an old rev of the nVidia drivers, or to
RMA the board and get a later revision instead. I haven't heard yet
whether either of these are solutions, but I'm continuing to research.