How fast should my internet be to play call of duty on xbox live?

Hi, I have dsl with at%26amp;t and I have the fastest internet package which boasts at 6mbps, and when I use only 1 computer and run a speed test on speedtest.net it averages at about 5.3mbps, but when I use more computers and use more bandwidth it drops to about 3 to 4mbps. All of that is expected and i'm fine with it, because my computers run relatively well, like for example if I watch a video on youtube it doesn't have to stop and buffer every 2 seconds like it use to with my slower internet package. But the problem is whenever I play call of duty on xbox live it lags atrociously like it used to with my slower internet. I tried everything like plugging it straight into the modem (I have a 2wire 2701hg-b modem router combo thingy if that helps) I've even tried that port forwarding thing and still no change. Is the problem that my internet is still to slow, or should about 5mbps be good enough to play without so much lag, or is the problem my modem? Someone please help because I upgraded my internet to play xbox live without so much lag in the first place, so i'm currently wasting my money. ThanksHow fast should my internet be to play call of duty on xbox live?
it could be u have to many stuff on ur computer.did u ever think about that?