Believe everything downloaded since the switchover is 256. Everything downloaded before the switch is 128. There was a period of time when Apple was charging to upgrade files purchased in 128 to 256, but I'm not sure if they are currently doing that.
Almost "everything period" with a few exceptions. Anything you bought before the switch that has been re-released as "iTunes Plus" was upgradeable to the higher bitrate for some number of pennies per song. Only really obscure stuff wasn't converted. In my collection I have maybe 5-6 songs that never got "upgrades" available and that was out of a couple hundred. I think it cost me like $35 to get the iTunes Plus versions.