arcade has no Hard drive, but comes with a 256MB memory card. you can go online with it, but obviously without a hard drive, you wouldn't be able to do much, plus since the lastest Xbox live update, it requires 128MB of space. so there goes half your memory card space.(it might come with the 512 MB one now, but I'm not sure). sometimes comes with acarde games. you can however buy a hard drive separely and attach it to the arcade 360.
Pro or Premium now comes with a 60GB hard drive(used to be 20GB) wireless controler, headset for Xbox live, and ethernet cable.
Elite. basically the same as pro, except it's black and comes with a 120GB hard drive, and HDMI cable.
other things that come with it might depend on where you get them. some have 2 games, 1 month free xbox live gold, etc...
http://www.xbox.com/en-US/hardware/compare101.htm