RUNNING AIR AND NITROGEN

what do i need to run both? and how is it set up? right now i have a basic 4 valve 2 switch set up... witch is fine... but i want alot more speed! am i correct in assuming that nitrogen will bring me speed?how do i go about adding this... including where to get a bottle... am i right in assuming that when i want nitro i can use it... then use air from my compressor other times? like just using nitro to play and air to cruise....