sup dude. to answer all your questions ill make it simple.

to join xbox live all you need is your xbox. an xbox live gold membership card which you can buy from any game retailer. a modem if connecting directly to your xbox with an ethernet cord with both ends in assigned ports. also you can buy xbox live subcription from your xbox dashboard using a credit card if you would like. yes, silver will stay under your gamertag even if your gold runs out, but trust me. you wont enjoy live if you dont have gold. depending on who or how well your internet is you shouldnt experience any lag. you will only tell if playing online. some games will show how well your connection is. like halo. and yes if others have bad connection it can affect you. you will lag. so hope you dont get stuck with someone like that.

i think that just about answers all your questions. hit me up if you have more.