Should a phenom 955 be necessary to run SC2 better? Considering you have a second-stepped high-end phenom, you should be fine. Instead of spending $160 on a new phenom, just grab a $40 heatsink/fan, and OC it.
IMO not cauz i can get SC2 on 1200p at ultra at a constant 60 fps without AA, cauz Full HD doesn't need AA, but will be doing 4xAF.
A question about replacing my current 3870x2 is how will my drivers work? Considering you already have the ATI CCC, just go to this page, and download the driver only (not Catalyst Control Center).
please explain how should I remove my current ones for new ones. See: above
Also what about rivatuner, will it automatically detect 5850? I though rivatuner was for nVidia cards only, but looking to the site, it supports any ati graphics card, so you should be fine.