Having never done a "Nut" beer....I would guess that you'd need to really crush your pistachios to get the flavor out. You might try mashing them, instead of adding them to the boil (avoid having to clean pistachio bits, and mess from all your equipment and tubing later), and still get the flavor. I've don this with pumpkin with success. Also I wonder what the effect of the natural oils in nuts would be on your beers head. Usually oils kill head. Good Luck!!!