This post is more a post for me, than a general post, but it could come in handy for others.
A while back, i got a Linksys RV042 for my home router. this was all well and good (2 internet connections, linked to give me massive bandwidth!). Its all great. but, my XBox 360 failed to connect to XboxLive. this started to annoy me, as i wanted to play games… Well, i found the issue. Its all to do with the MTU setting on the Linksys Router.
To fix it, login to your router, go to the firewall tab and set the MTU setting to Manual, 1365. i changed the setting, saved, and XBox live loads first time! works great!