memory_max_target , memory_target question ?

  now the customer  have 11g (11.1.0.7) in suse linux,

have 8G RAM,

they find  a things is that  memory_max_target ,  memory_target  parameter setting?

setting 1:

memory_max_target =4000M

memory_target  =4000M

startup db ok.

 

setting 2

memory_max_target =4800M

memory_target  =4000M

startup db failed

ora-03113 error

 

setting 3

 memory_max_target =4200M

memory_target  =4000M

startup db ok

 

this seem memory_max_target  and memory_target setting ,what's the correct setting?

can any explain why did oracle use memory_max_target  this parameter,

i think setting  only  memory_target is useful and ok.because oracle will use memory_target to allocate SGA and pga,