HELP needed seting up wifi network

Is this house on campus? Using the college's internet connection. If so, then the first step is to talk to your network admin(s). They decide what happens to their network and usually wireless (beyond what they control anyway) is a sticky situation. Inherently insecure and without them being able to control security then chances are it will be a no go. I would certainly not allow it as an admin.

Beyond that, 802.11 a, b or g is not OS dependant. You just need drivers for whatever card you have, be it in a MAC or Windows box.

File sharing gets a bit trickier, especially if you have a mixture of MAC OS's, meaning OS10 and pre-OS10. OS10 you'd just setup samba server and client. Before OS10 you'd need to run something like DAVE to file share with windows machines. With internet OS does not matter except for the aforementioned driver issues. Same with gaming. With 40 people your going to want multiple Access Points or your pings on games will be pathetic. Even with 10 people on an AP the 10th person (or whichever is gaming) will/could spike dramatically. AP's are shared bandwidth. Akins to hubs. For messaging, just use Messenger, Yahoo or Microsoft, or AIM, Trillian etc... Whatever you'd use for internet messaging. I assume MAC has a Yahoo messenger port so that might be the easiest solution.