[Asterisk-Users] How to reduce asterisk CPU-LOAD?

[niels at wxn.nl]

Hello I have a Dell Poweredge / Dual 3.2 GHZ XEON / 2GB ram running
asterisk

It's configured using realtime-extensions / sipfriends / iaxfriends (to
a local mysql daemon), 80% of all calls are IAX <-> SIP calls with no
codec transcoding and no jitterbuffering, and 20% of all calls are IAX
<-> IAX native transferrred calls, again no codec translation.. the box
has NO zap channels

Now, when I have a load of 100 simultaneous users calling (that means I
have 200 simultaneous lines open of IAX and SIP calls (where 80% of
those calls are IAX calls translated to SIP/RTP) then asterisk uses an
average of about 30% to 35% processortime (divided over the processors)
and mysqld then using about 1% average load .. so the maximum callers I
can have is about 300 .. 

This CPU load is that mostly generated by handling all those callsetups
or is it mostly generated by the IAX -> SIP+RTP proxying(translation?)
and some IAX -> IAX native bridged calls?

how can I finetune this making one box to handle more simultaneous
lines?

will faster CPU's work? (3.6 ghz won't help much huh :)
will 64 bit cpu's work? (The ones I use now are 32 bit xeons)
will MORE (4) CPU's work?

will something else work?

is this ONLY a cpu problem or is there more I have to take a look at??
Memory speed etc?

all ideas apprecieated.

Thx. Niels