I'm not quite sure why you would have both a router and wingate, but you are basically going to need to make sure that outside clients will be able to get to the server. If the P200 is acting as your router, then you shouldn't have much of a problem because people will be able to connect directly to it. If you have one of those linksysnetgearsmcdlinkhawking dsl/cable routers then you will need to forward the proper ports to the P200, namely 27950-2 and 27960 udp.
As for choice of OS, Win95 isn't the best, but having 80 megs of ram and only a 4 client max shouldn't pose too much of a problem for it. If you can somehow get ahold of NT4 Workstation, do so because it has a much better networking stack than Win95 and might even result in ping times of a few ms lower. It would also allow you to remain under the shelter of a windows gui. Also, I've been able to tweak NT4 Wkstn so that it uses < 16 mb of ram at startup. Of course the optimal thing to do, provided that the P200 would never do anything but serve Q3, would be to throw some distro of linux on it and learn a bit.
But anyway, what you have now in terms of hardware and software should be just fine to run a 4 person server. Just don't expect to use the computer while it is running the Q3 server.
Best of luck,
~bex0rs