No matter what your speed of Dongle, you will only get the maximum of what your tower provides you. MTNL has towers capable of 3.6 mbps speeds. However, someone on a different forum says that MTNL is silently upgrading to 7.2 mbps and is testing it in some areas like Andheri East.
He also provided a screenshot of it. I don't know if MTNL is really testing higher speeds or not, but until they make an official statement about it you should expect 3.6 mbps, not more. And when (if) they upgrade their speeds then you should be able to get higher speeds as your dongle supports upto 7.2 mbps. BUT if MTNL decides to go even further, then your dongle will be a limiting factor!
http://img691.imageshack.us/img691/5073/9695d1313782148mtnlmumb.jpg
PS: Credit of the above image goes to the original uploader.