4Mbps is more than enough. I play games on PS3 comfortably... BTW its not the speed that matter in online gaming, its the ping and consistencry of service your ISP provides. If you want stable and moderate pings go for Airtel. If you want low ping go for ACT broadband if its available in your region.
As others have said - bandwidth is not that important after say 512kbps. What becomes really important is the ping - and not the ping you get to Indian servers (unless you are playing on an Indian server with other Indian players, which is really only feasible on PC, not consoles (less flexibility to choose servers). So in consoles like xbox 360, it's the latency that will be the single biggest factor. In this aspect, Airtel sucks majorly. I am not sure which ISP is better, but w.r.t. latency Reliance, MTNL are supposed to be better.
I play Call of Duty 4 Multiplayer and the server is in france and I get ping of about 200-230ms and I played it with 256kbps, 512kbps, 2mbps plans.. speed doesnt matter.. I always score max and ranks number 1