And the answer is . . .
ASUS wrote back with "137GB maximum". A tech at the local computer
store says that's the limit for the current EIDE addressing scheme,
and that a larger HD can be used as long as it's broken into multiple
partitions (each of which must be smaller than 137GB), at least until
the next addressing scheme is standardized; although some boards and
BIOSes have various attempts at the new scheme, they don't work all
that well, he said. (With thanks to Iron Woode, ASUS, and the guy in
the suit at Computer Stop in Bellevue, WA.)
So I guess it's safe to buy that 150Gb drive after all (as long as
I set up a ~15GB second partition). (I guess I should feel guilty
for not buying it from CS, but OTOH I bought everything else there!)