WoW only consumes 56kb/s I've got 3mb/s Down and 1.5mb/s up. supports 2 people playing wow and then some. However, I still CAN lag on occasion and on different games. like in diablo 3 I get 120-400 latency as my average.
Your best bet get a simple 50mb/s connection. that seems relatively standard for multiple people.
Then comes into question that many people running. 2 running gaming. make sure you get a good modem/router. I am stuck with Verizon and I've got through 6 routers over a 2 year period so far. I now got one of their new models of d-link routers and it seems okay as they all do at the beginning. But, running that many people and gaming and downloading whatever it may be will stress a router a bit.
I haven't looked into this into technical detail but also try to have some kind of fan on the router/modem it seems mine would get hot and performance cut in half.
So, sadly you could get 100mb/s download speeds and still lag or have terrible speeds compared to what your paying for. many things rely on Location, service, time of day/week to determine what you'd need. However, the best thing to do is determine what every computer will be doing. HD movies require a lot of speed. I can't run any HD movies with my network ever without having to spend 15minutes buffering.
Basically you'll divide advertised speed by 10 and that's what you'll get commonly. Even less when during high times in the area for the isp.
Edit: I can recall my father had Comcast in PA. He lives in the country and he got I believe a 30mb/s service and it couldn't seem to surpass downloading at 500kb/s Total. recorded speeds for the speedtest.com where about 5mb/s. He tried to call and many other things rewired part of the house and nothing fixed it. It came down to the factor that well he lived in the country and internet in the country is always half the quality as it is close to a city. comcast well they wouldn't do *** about it.