• I would expect that cpu usage would increase when you start to load the server (wasn't sure what your point was there)
• RAM usage depends a lot on server configuration and also on number of plugins/what plugins. You will see different RAM usage patterns Apache vs nginx.
• Make sure you are running the latest version of Elgg
• Make sure you have all the caching turned on for Elgg
• Make sure you are using something like APC
• Search plugin hits the DB extremely hard

I hope you realize that hitting a server like that is not at all an accurate picture of how many users you can support. It is a worse case scenario - slashdot effect kind of analysis.

which server crashes ? #1 or 2 ? 100% ram usage ! does not sound right ;(
we've had 150 simultaneous users some 2 years ago.. with no problems except occasional 'warnings' or slower responses with similar server hardware.
one small piece of advice for this situation.. take from our experience
the fine-r tuning that we did not to get to that state was not achieved with short questions and quick answers -- a fair lot of apache, mysql, linux configuration kopw-how and.... a lot of patience over time to monitor performance and config changes' resultants was done before the server showed improvements. not a task for weak-hearts ;-) when the server behaves unexpectadly and requires very quick restores to config while scratching heas to deternine what went wrong..