I'll try to give you some answers - I see you guys are really thinking this through and deserve some technical information:

-It's not my domain to tell you if big bandwidth hosted servers are a 100% certainty... this is something that Xbox and Atari need to work out. Last I've heard this is going to happen.

-Bandwidth. We're hoping that people want to host games and I've been working for several weeks on trimming the bandwidth utilization. When you host, the game will determine what your bandwidth is capable of supporting and limit player counts accordingly. "5000 bytes per player AT LEAST" is very dependent on the context. If you can keep the roundtrips (ping) low and reduce redundant information being sent than 5KBps is not required - likewise if you send tons of information redundantly 5KBps may not be enough! With UC we've put alot of effort into making the packet stream as efficient as possible by removing redundancy (i.e.: if you see a bullet impact, the client implicitly plays the sound, rather than sending a 'playsound' packet). We'll be working on this until the very end. UC is using ~2KB/player right now.

-Voice. Currently our voice data is about 1KBps/player. Voice data is channel based to avoid the packet storm that would result if everyone could hear everyone else. The voice bandwidth usage is very tunable and we've been experimenting with latency/bandwidth tradeoffs (the packet header overhead is significant). There are 4 players in a single channel, and you can easily change channels. Today we're testing a 'must push a button to talk' to see how it plays - this is a good bandwidth optimization because its more consistent than silence detection throttling.

-128-256kbps is exactly what we are testing with. Currently the target is to support 8 players nicely on a decent DSL-based listen server (where the host can play also). If you have a bad-ass connection this could go higher. Again, the goal is to have decent games with reasonable player numbers.

-Multiple players on one Xbox. It merges the bandwidth requirements of the two players, so best case scenario is it uses only ~10% more than a 1 player client. Playing splitscreen multiplayer rocks!