pretty much anything over 512kb is enough, but speed isnt the main factor for gaming. ping time is king in gaming, 10mb with a ping of 300ms will pale in comparison to 512kb with a ping of 60ms. unfortunately theres no way to tell what the isp's ping to you particular location is without actually having it installed and trying it.
The question is valid, and lots of people mix up speed and bandwitdth. A bandwidth of aroung 256k is actually sufficient in current gen MMO's. The key is indeed speed (latency). You want a latency that is as low as possible, but absolutely no higher than 100ms (which is already high for online gaming). Depending on the server and client load and local network setup (your home) and any other downloads running in the backgroud a 100 ms ping to the gameserver may translate to a latency of 200 to 1000 ms.