2mbps is the lowest I'd go, but you should aim for 10-15 for pretty much seamless playability and quick load screens.
Other than that anything in excess is really for downloading purposes only as when it comes to online gaming it's more about how quickly your computer can load the sprites than the speed of your connection once you're past a certain amount of mbps.
I used to have 10mbps and now have 30mbps and I honestly cannot tell the difference since my hardware is still the same...
"Bright and cold, the flash of steel,
Alone I roam over desolate fields,
My sword, I no longer need,
The evening wind will carry me home."