Choosing how to approach Geocoding Requests

Question

I am about to begin writing a program in c# that will read Addresses from a source file create a Geocoding request, sent it to Google Maps API, get the response choose the coordinates from the xml and then store them in a database.

My question is what is the proper type of source file considering performance and easy implementation.

I have the option to supply the source addresses as txt file or xml file. The number of addresses need to get geolocated is about 100.000 which is a big number so what is the proper approach to handling this kind of requests? Should i provide the addresses source file as txt or xml or something else?