Need for help!

Dear all,
I have a postgres 8.2.5 and ~6 GB database with lots of simple selects using
indexes. I see that they use the shared memory so much.
Before, my server has 4GB of RAM, shmmax 1GB, Shared_buffers is set to 256
MB, effective_cache_size 300MB, when i test it's performance with option -c
40 -t 1000, it's results is about 54.542 tps, but when i up number of
clients to over 64 it refuses to run? Now, my server has 8GB, shmmax 3 GB,
shared_buffers is 2GB ----> it uses ~ 7GB cache, after benchmark ( c 40 t
1000 ) the results is 57.658 (???). But after upgrade the max clients is
also 64 (?!?) Is this the maximum clients support by program pgbench (my
server on Linux ver8.2.5, pgbench on Windows - version postgresql is 8.3.1)?
And the number 57 tps is fast?

Another questions, i heard that PostgreSQL does not support HT Technology,
is it right?

Last question, i don't understand so much the shmmax, shared_buffers, after
upgrading my server from 4 GB RAM to 8 GB RAM, first i configure shmmax to
2GB, share_buffers to 1GB and start server, it runs, after that i set shmmax
to 4GB and restart, it fails (?!?). The error logs said that not enough
share memory! and final i set shmmax to 3GB and share buffer to 2GB, it
runs. Don't know why, can you explain?
Thanks so much!
Regards,