Microphone to use with OS X's Speech features
I am not sure the iPhone headphones with the built-in microphone will work on older MacBooks, since I believe they do require a headphone jack capable of sending the microphone input. Headphone jacks on MacBooks shipped after the iPhone was introduced all have that capability. 

As fas as microphone recommendations, the following two seem to be regarded as very good by different sources I trust, such as the Equipment Guide that 5by5's Dan Benjamin writes every year: 


For beginners: Plantronics .Audio 655 USB Multimedia Headset
For pros:  Heil PR40 Dynamic Microphone


Hope this helps.