Well, I have a number of answers for you: First off, yes that is good enough for Xbox Live. It should be excellent (I have 150kB/25kB) and I have no lag is no one is downloading anything on the network. Of course it also depends on the location. Assuming that your in the North near Canada, you could experience lag with host that are in the westward states, the South and most definitely Japanese host. Otherwise, you should be sitting pretty.
You should always check the forums, and the sticky's posted on how to improve your connection! That's what this site is about!
Good Luck and Hap