Help :Mental ray fatal error

Hi every body.
there is a Fatall error porblem for me.
I have one location in my project.it is a street.fully detailed and I have about
8 characteres.when I rendered  a character and the location seperatly.I tried to
test character in location.now there is another big problem : " Fatal Error
!! " 
system is out of memory!!
and can not reder any thing.
my scene with one character has about 2,000,000 polygones.
this is for 1 character.location and paint effects trees that have been
converted to mesh(as you know because mental ray can't render paint effects!!) 
in render log maya tells that out of memory. in the task manager only 1.7 GB of
memory used my performance is :
4 GB of RAM
4 GB of virtual memory
3.0 GHz AMD Dual core
Win XP 32 bits
maya 8.5 sp1 32 bits
and memory limit of mental ray is 2048