I changed something and got:

cg5:~/cs/rb/ruby quizzes >time ruby 90cheat.rb 4000

real    0m37.412s
user    0m31.385s
sys     0m1.846s
cg5:~/cs/rb/ruby quizzes >time ruby 90cheat.rb 10000

real    10m56.151s
user    9m8.446s
sys     0m30.292s

For 10,000 i saw in Activity Monitor it claimed to be using 1.7 gigs  
of real ram, and another 1.7 of virtual at some point near-ish the end.

Elliot