Sorry for the long post. In summary, upload bandwidth is often not the most important factor in determining who is the best host.
There are two factors that contribute to whether you make a good Live host. The first is upload bandwidth. Upload bandwidth is important as the host needs to be able to send the every player the data of every other player. If the host doesn't have enough bandwidth, then everyone lags like hell and players tend to be dropped by the game. But if the host has enough upload bandwidth, having more doesn’t help.
So for a Team Doubles game, with only 4 players, upload bandwidth is not going to be the most important factor in determining the "best" host, as everyone is likely to have enough upload to cope. Upload bandwidth will come into play with a 8v8 game though.
The second, and in many cases most important, factor that contributes to whether you make a good Live host or not is latency (or packet delay). This is often misunderstood, because lots of people confuse bandwidth with "speed". So they think a "24meg" connection is fast. Leaving aside that a "24meg" connection refers to your download rather than upload, more bandwidth does not mean lower latency.
Latency is how long each packet takes to get to you, and is usually measured in milliseconds (ms). You might have a huge bandwidth (the amount of data you can receive per second) but a high latency (how long an individual piece of data takes to get to you).
An easy way to think of this is to consider the postal service as the Internet. The amount of letters your postman can carry is your bandwidth, and how long it takes him to get from the depot to your house is the latency. You might have a postman that can carry lots of letters but travels to your house via a long route (high bandwidth, but poor latency), or your might have a postman that can only carry a few letters but runs to your door (low bandwidth, but good latency).
For a small Halo game, it isn’t the amount of letters the postman can carry at once that is that important, but how fast he can get them to you is. For large games (8v8), the amount of data being transmitted at once becomes most important. High latency may be the fault of your ISP (some are simply faster than others at routing stuff) or it may be that you are physically far away from the other players. Electricity takes time to travel around the place. For example, your data will always take longer to get to the US than it will to get to someone in the same city.
I made a thread for this will people posting their results over at NTSC-UK. Thread can be found here:
http://ntsc-uk.domino.org/showthread.php?t=71173
The interesting thing about those results is that there are quite large variations in latency. In fact the person with the highest upload bandwidth, has quite a poor latency to both the UK and the US as he is based in Norway. Conversely, a bloke with an upload of only 188kb/s has great latency to both the UK and the US. For a small Halo game with other UK players, the guy with the much lower upload will make the best host.