Check with your local 'car stereo' installer store.  Most towns these days
have at least a couple of these places.

When I bought my last car a couple of years ago I dropped in to see what
they had available since my new SUV didn't have a cassette to use the old
silly adapter.  I had tried the little battery powered FM broadcaster with
less than stellar results...

The perfect solution is a device which plugs in to the line between the
car's antenna and the car stereo.  This is essentially an FM broadcaster
that is hardwired to the radio so there is no interference.  The thing gets
set to a semi-permanent frequency (which can be changed if needed) and comes
on when a dashboard switch is activated.  It connects to an audio cable
which comes out from under the dash and coils nicely on my console tray when
not in use.

So, when I want to listen to audiobooks, music or any other sound output
from my iPAQ I simply plug this cable into the headphone jack, tune the
radio to the preset channel and flip the switch to activate the system.
Viola!  Instant, crystal-clear sound with absolutely no interference.
Since the FM modulator (believe that's right) is wired into the antenna lead
there is no leak-over from local broadcasts in nearby frequencies.

This whole setup was right around $90-$100 and was the best accessory
investment I've made.  Since the cable is tipped with a standard headphone
plug it will accept anything you might use regular 8mm headphones with
including MP3 players, PDAs etc.

I'd highly recommend such a modification.  And, best of all, since it simply
plugs in to the antenna lead (plus a powered wire from the vehicle) it would
be relatively simple to transplant into another vehichle should you sell
your car.

$0.02

Terry

Entering this thread a little late but we have reviewed the Sedio mounts
which have built-in amplification.  Personally I use a gadget that was
designed to amplify output from PDA GPS systems, see
http://www.pocketgpsworld.com/qubit-sound.php which is superb and can even
be wired to mute the car audio when GPS navigation instructions are being
given.

As for mounts, the best by far are Brodit's, see www.proclipusa.com for
details and our site for reviews.

I have the same GPS system with my IPaq 4150 -- I have the same
problem with the sound, and I like t listen to music.

For $20, sony sells a pair of amplified speakers (I got mine at Best
Buy).  There is one that has power, and a second that plug into the
powered one.  They work with or without batteries (they are passive
without).  Here's the key:  Either works without the other.  I use the
non-powered one in my car -- It's not really loud, but it's MUCH
better than the PDA alone.  I use the powered one on the road when I
travel -- It's MUCH louder, which is better when I'm in a completly
new area.

Hope that helps!