[Hectic]

not sure where yer located but around here 350fps is the max for indoor.
in alot of cases if you lock it to semi you can run 450 outdoors.
as to your actual questions, most hage been covered.
but ill run a list.
good inner barrel
good hop rubber (bridge hop, flat hop, rhop)
good cylender head
good piston head
a good spring isnt really needed if the stock one is pushing tue fps you want, however a good spring should be more consistant, and maintain fps for longer then most stock springs.
good bbs, the heaviest you can use, likely .30's if yer gun shoots 400.