we are developing a system that tracks the locations of several users. The users upload their locations at a server and then these locations are stored in a database. For now the database is MySQL. We want to make queries like "tell me which users are at a specific square or circle" or "tell me which 5 users are closest to user Tom" etc.

I have the basic knowledge of databases and MySQL but I am not specialized in databases, so as a first thought was to answer the above queries by using MySQL queries and some maths. With a small search I found that there are db systems like PostGIS that include many operations that we need. Especially for PostGIS there are many programs that use it as backend like Quantum GIS that we will be for me very convenient. 

My questions now :P

I read that PostGIS is an extension of PostgreSQL, that means that i can use at the same time PostgreSQL for manipulating other data (e.g usernames, time etc) and PostGIS to manipulate the users' locations or I have to create two different databases?

My deadline unfortunately is very tight... So it is easy to pass from MySQL to PostgreSQL or not?

Any other ideas are welcome!

Thanks in advance!