I’ve switched to nginx recently too and it’s sweet!
I see major difference in memory usage and my server is much more relaxed now.
In the past you did various tests with ab, could you please tell us how it works with your nginx setup?
I run ab -c 100 -n 1000 on my server and
without SuperCache I get 22 requests/sec
with Supercache I get from 530 to 620 requests/sec
and on static files (gif,txt etc) inside my WP blog I get from 3100 to 3600 requests/sec