I would like to respond to those who are looking for a method to estimate equipment in a GSM network. You need to provide some additional information. Mainly, what is the Erlangs per subscriber and the number of subscriber you are expecting per cell, BSC, MSC, etc. Once we have that we can start to estimate the amount of equipment needed.
To get an idea of what is needed there is a white paper that I have written at http://www.awardsolutions.com on high level wireless data network provisioning (i.e. GPRS or CDMA2000). Feel free to take a look at that.
If you would like to discuss this more, please feel free to send me an email (firstname.lastname@example.org).