Tim:
What is the width and depth of the theatre? Two groups of 5 seats 20 deep? Two groups of 10 seats 10 deep? Are the seats rising as they go back? Is the stage elevated?
What resources, books, DVDs/Downloads, effects, etc. do you currently have?
What effects do you currently perform?
What magicians do you like their performance?
What is your character?
What is the typical audience of the theatre?
How long is the show going to be?
What is your budget?
All of those factors go into selecting what to perform. What I would perform and what you would perform probably are completely different.
Now, I am of the school of thought that you shouldn't mix mentalism with magic, even geek magic UNLESS you can make a coherent connection between the powers displayed. That is, what is the connection between being able to discern a chosen word from a book and being able to swallow needles?
For a mentalism show, I would do the following:
Lie to Me - There is one person in the audience who is lying to me and I find them by process of elimination.
Silk Stocking Murder Mystery - My own routine where the spectator solves a decades old murder based on an old box containing the evidence of an investigation
Psychic Scrabble - Where I discern a word the spectator makes out of a random selection of Scrabble tiles
Identical Impossible - A drawing duplication where I draw the exact picture drawn by the spectator.
Romance - A routine where I discern a word from a romance novel as well as the spectator's thought of romantic dream date.
The Psychic Trust - A bank night routine where the I work with a spectator to save $100 bill from being burned.
Amulets - After explaining the significance of each of six amulets, a spectator selects one of the amulets from the top tray of the jewelry only to reveal a correct prediction under the tray
Lettie's Feather - A one person Touching on Hoy routine about a Jewish mother and son that are separated when sent to Nazi Concentration camps.
Credits (in no particular order and put at the end to make it difficult for anyone to reverse engineer my routines) Jim Steinmeyer, T.A. Water, Max Maven, Rich Ferguson, Walt Anthony and Craig Browning.