In terms of upstream and downstream, about how much bandwidth does Xbox Live require for games like Black Ops, while using the VoIP feature with friends?
If you have DSL, you don't need to switch providers. The best thing you could do is request your DSL provider to remove interleaving from your line if it exists. This will give you what they call a "fast path" connection. Fast path reduces lag or ping time by as much as 30 milliseconds because it bypasses some error checking of the data. If you ask for it and they say your line isn't stable enough then tell them you want a tech out to adjust your connection on their equipment (somewhere in you neighborhood). Once your line is optimized, they will remove interleaving at your request.
The bandwidth requirements are so minimal that you should not even think about it. Your average connection (DSL/Cable) will be able to handle it no problem. If you are on a slower connection, can you use other XBOX Live features? If yes, you will be safe to try gaming too. The games don't actually send/receive much more data than the dashboard believe it or not.
If you want hard numbers, the only way to know for sure is to use a network monitor between your Internet connection and XBOX. This will give you exact measurements of the requirements. Be aware though, this will differ between games, game types, and other factors like number of players, whether you are hosting the game or not, and if you are using other XBOX features like Party chat and such.