exim configuration

I am in the process of moving away from qmail to a more ISP friendly
MTA (qmail opens one connection per email). Ive been looking at exim
and postfix. I had postfix configured pretty well, but when I added
the domain keys "hack", postfix crawled.

I have turned my attention to exim (much better current solution to
domain keys). In internal test, it ran great. It delivered emails to
other internal email servers pretty fast. When I tried a real world
test it got backed up and the queue just kept growing. I tried to send
an email to 20k emails and the queue got up to 19k+.

my latest set up has:

split_spool_directory
queue_run_max = 5
remote_max_parallel = 40

in cron I have
/usr/local/exim -q running every minute.

I use dnscache on the server and it runs Red Hat Linux.

emails get added exims queue through an smtp connection.

Are there any tips as to what I do to speed exim up?
Does exim require a high amount of memory to run fast (the server only
has 512 MB since qmail can run very fast with less than that).

thanks,

Isaac