Blakeyz, are they all confirmed by RSVP? If so, you should expect a few not to show up, and cater for about 160. If that is just an estimate based on how many you invited, I would say cater for about 140 - there are never as many people as you think there are going to be.
Of course, it IS better to have too much food than not enough. If you're not worried about money and you don't want to stress about having enough food, cater for 170 and get people to promise to take some home at the end.