1) they use win7, which in general would require 1gb ram for each vm; but you can use micro7 too reduce this (official it needs around 140 mb on a clean install, so with background stuff etc i would suggest going for 256mb or 512mb) and yes, you will need win7, cause .net4.5 isnt running on XP
2) the new gibot doesnt use the Lol.launcher itself, it handles login, que and matching from DOS / Shell / terminal (however you call it); usually the client takes about 300mb for itself and is opened all the time (even if you are in a game, you can open the client to chat / etc, its just minimized)
3) the game itself doesnt seem to require this high amount of ram, i just created a 1+5bot game on my normal acc / computer and the game itself took about 800 ram but with max'ed settings in 1920x1080; ( lower resolution and settings will cut the ram by ~75-85% i guess)
4) about your disk storage: you can just create 1 vm, setting everything up, making sure everything works and then clone it, but clone it as "linked", not as "full"; this will make your clones use the same datas and you will reduce your Storage to a min
5) assuming the AMD FX is a octa-core, i would assume on microXP with usual bots ~20-26 (more and its getting instable)
with this method i would assume, you can handle maybe 30-35, i guess your problem could get the core, not the ram (or your internet bandwidth)
if you need more information, feel free to send me a message
@op, you can use my text for your FAQ, if you like to, feel free to modify it aswell
edit: but keep in mind that riot gets the login ips and if you run so many bots simultaneously, i would suggest getting some proxys to use for your virtual machines