upload speed is most important. i just upgraded from dsl to cable on the advice from xbox live support because I could only get .75mbps upload speed with the dsl and apparently the minimum for good gameplay is 1 mbps up. So now I am 3mbps upload and have 4bar in darn near every game. Hope this helps
Upload is really only important as even some of the lowest Down speeds cap out the game.
3mbps is close to soft cap and I think 1.2 is soft cap for up with 1.5 being optimum (as you always want to have some leeway incase speeds drop).
Download really means nothing if you are over 5 megs download speed. DL speed is really only for streaming, downloading etc.
Upload should be around 2 or 3 mbps. You can get by with lower but that doesnt give you any cushion for error.
I have 1.7 as per my speedtest on the PC and that works fine.
One thing that really helped was to change my router to a N based connection. Now both Xboxes play without issue in MW3.
Ya thanks for reminding me! I did the same as starbuck bought a nice router and bridged it to the modem as well as the switch from dsl to cable. Now both me and my roommate always have open nat type, and 4bar. Cant wait for a few months from now when they upgrade their system I will have 5mbps upload so I am hoping the few little hiccups that I still have will be gone after that