I've heard polling the server is not the best of ideas.
Let's say I make a client-server application. A simple game for example. Where each client polls the server every half a minute. How many clients is it possible to have before it overloads a wamp server? Basically how robust is Apache for this kind of stuff? Getting a request, aggregating data from mysql server, and then returning the data in an xml format.
This is a really open ended question. It entirely depends on your configuration, how many apache services do you have running, how many physical servers do you have, how is your mysql server setup (is it on it's own machine)? You want to also keep in mind that by polling the server you have to initiate a connection each time and allocate resources for that communication (in the lower level networking and your program).
If at all possible it might be better for the server to push content to the client (assuming a push happens less frequently than polls happen).