I just wanted to give my input also, I would also recommend a Linux Server.
On Ubuntu Server, 12.04LTS, Moodle uses anywhere from 50mb to 100mb of memory per concurrent process during a quiz, the lower end is just forums posts and other clicking around,
The attached is live reading during a quiz, Moodle, no-addons, secure quiz in popup, 1 question per page, you can see how much memory each concurrent process is using.
Using the highest for today, that would be 10 concurrent processes per GB of memory, this is what I have always seen and in all my posts. 4GB of RAM = about 40 concurrent requests if all 4GB are dedicated to IIS or Apache, the OS would still need additional memory