so u have been laggin really bad online? have u port forwarded ur router? here r instructions for anyone needing to port forward you router.
1. find your model number of your router (ex. mine is Dlink EBR-2310)
2. find your router on this list http://www.portforward.com/english/routers/port_forwarding/routerindex.htm
3. click on xbox live 360 link
4. first make sure u have static ip on your 360 ( first paragraph there is a step by step guide on how to make one)
5. go back to the previous page and follow the steps to portforward your router
i personally did this and it fixed my lag. and before i couldn't voice chat with my friends it also fixed that.
ps. on p2p games u could still lag because ur running off of their connection. so if u still lag on p2p games (ex gears of war) that is y. also make sure u don't have multiple firewalls on ur computer (interfere with each other) and add xbox live on ur exception list for your firewall