Guessing game: G5 RAM installed asymmetrically; consequences?

[jimbrady]

My neighbor is having some strange behavior in his G5 tower--in poking around, I found that he had his RAM installed wrong--4 sticks in the bottom set and two sticks in the upper set (my first clue was that the profiler claimed only 2.5gb when there should have been 4.5).

I got the RAM arranged correctly (and Sys Profiler now sees it all), but this other problem still exists.

Any guesses what problem I might be seeing? I'll give the answer in a little bit, just trying not to skew the guesses.