You don't need sat nav in NYC. Use a real map. The city is a grid and (most) streets have numbers, one must be really dumb to get lost. Buildings are likely to kill the GPS signal anyway. Plus most people speak English... You'll probably walk or take a cab (good luck finding a driver that speaks English), I would discourage you from taking the subway unless absolutely needed, the system is very convoluted and lines are suspended all the times, plus you have express trains that don't stop in all stations. If in doubt always ask.
Be wary of the electronics stores and keep in mind that the sales tax is under 10% (actually 8.875%). I've seen instances where the $130 item was suddenly becoming $168.24 due to "tax".