As you found, for all the memory OW takes, and all that you had left, you still had to force quit it ;)
It's
http://apple.com/getamac/ads/ that consistently crashes SP7 :) I can now say that it will do this while there is no paging going on, and no use of the swap, and while there is still plenty shared memory. If I'm lucky, and it doesn't trash the native java, then I can report the crash and force quit, but the memory that was consumed by OW is not released back into the pool, obviously, that can only happen so many times. Don't worry, I'm supplying the necessary feedback by email :D
This is a fresh install of 10.4.6 (not done as an upgrade) with 512Mb memory. There are no reported fsck problems, no problems reported by disk util (not even a permissions problem), and no other instabilities from any other applications. I can trigger it even if OW is the only application I launch. I'm running bind, that is, I have my own DNS configured, as well as having made numerous networking kernel parameter changes, otherwise, it's largely bulk std. Also, Quicktime is Pro 7.0.4 (I mention this since the ads are movies).
For me 5.5 SP7 is better than 5.1.3 because it supports layers and allows me to edit Apple Groups.
In terms of stability, and memory footprint, I see no improvement, and those things have now got to be a priority.