Is your router detecting the connection? It could be that you entered the IP or gateway incorrectly, or missing something required for entry. Make sure your router's firmware is up to date as well. Also what router are you using, and what specifically is the fail message that live is giving you. Also try using the automatic setup on your 360, that may cover something you missed. Make sure that the IP you're entering isn't also in use by another machine (the router or a computer) as two machines cannot exist on the same IP.
I can't really tell a difference as far as it slowing me down any.
But yeah, as stated, you need to get a router & the plug an ethernet cable into your xboc & router, or you could get a router & use one of those wireless things for your xbox (that's what I do & it works fine).
that doesn't answer my router question. If you're not using a router and connecting it directly to the modem itself, I advise contacting your provider. Otherwise there's not much I can do without screenshots of your router settings, and your 360s network tab.