Hi Robert, I like your overall design, etc. I have been working on a Geiger Shield for the Arduino which makes designing a custom Geiger counter very easy to implement. The shield takes care of generating a variable high voltage to work with just about any Geiger tube and has buffer circuitry to interface to the Arduino for pulse detection. The is also an onboard LED and speaker which the speaker can be fully controlled by the Arduino.
I also have a 4 part Youtube video demonstrating the shield starting at part 1 here
I can also make the PCB available for those interested in building it.
Thanks, Tony