Greenpeace can sustain on the order of 100 hits a second (probably about 20-25 dynamic pages a second) with two single-processor AOLserver PCs (PIII 733s or something like that) and a Sun E250 running Oracle (a pig that any decent dual-processor PC would pound into the ground.)
The site's highly dynamic but we do some custom caching of content.
Make sure you have verbose turned off in your logs, that you have sufficient buffer cache defined for PG, that your OpenACS server is in performance mode, etc before measuring speed.
And as Jeff's said that you've vacuum analyzed your database.
What's your hardware?
