Some time ago I was developing a medical instrument which required histogramming, which got me in the mood to retake my own PIC MCA project(http://home.comcast.net/~prutchi/index_files/scint.htm ). I used the variable RAM in the microcontroller (16F877), so I limited the number of channels to 95 and let the histogram run until some channel reaches 240 counts (the highest 8-bit number that yields an integer when divided by 8). The firmware then displays the spectrum as a bar with a maximum height of 30 pixels for each one of the 95 channels.
Click here for complete how-to construction instructions in pdf format.
Click here for a pdf of the schematic diagram for the front-end of the MCA