Would my postgresql 8.4.12 profit from doubling RAM?

Hello,

I run CentOS 6.3 server with 16 GB RAM and:
postgresql-8.4.12-1.el6_2.x86_64
pgbouncer-1.3.4-1.rhel6.x86_64

The modified params in postgresql.conf are:
max_connections = 100
shared_buffers = 4096MB

and the pgbouncer runs with:
pool_mode = session
server_reset_query = DISCARD ALL;

The main app is a card game with 30-500
simultaneous users for which I save some
playing stats into the db +
PHP scripts to display those stats again.

I have an option to double the RAM for EUR 180,-
but wonder if it will improve any performance and
also what to do on the PostgreSQL side once
I've doubled the RAM (like double shared_buffers?
but how do I find out if it's needed, maybe they're empty?)

Below is a typical top output, the pref.pl is my game daemon:

top - 13:40:30 up 21 days, 5:11, 1 user, load average: 0.61, 1.14, 1.31
Tasks: 232 total, 1 running, 231 sleeping, 0 stopped, 0 zombie
Cpu0 : 14.6%us, 0.3%sy, 0.0%ni, 84.4%id, 0.3%wa, 0.0%hi, 0.3%si, 0.0%st
Cpu1 : 3.0%us, 0.0%sy, 0.0%ni, 97.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 : 7.3%us, 0.0%sy, 0.0%ni, 92.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 : 7.3%us, 0.0%sy, 0.0%ni, 92.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu4 : 10.0%us, 0.0%sy, 0.0%ni, 90.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu5 : 2.3%us, 0.0%sy, 0.0%ni, 97.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu6 : 0.0%us, 0.3%sy, 0.0%ni, 99.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu7 : 1.7%us, 0.0%sy, 0.0%ni, 98.3%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 16243640k total, 14091172k used, 2152468k free, 621072k buffers
Swap: 2096056k total, 0k used, 2096056k free, 8929900k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
19992 postgres 20 0 4378m 782m 775m S 11.0 4.9 2:28.58 postmaster
16184 nobody 20 0 116m 21m 3908 S 9.0 0.1 22:01.32 pref.pl
16187 postgres 20 0 4375m 502m 497m S 7.3 3.2 37:13.48 postmaster
20229 postgres 20 0 4377m 426m 420m S 6.3 2.7 0:07.01 postmaster
20201 postgres 20 0 4378m 512m 505m S 4.7 3.2 0:23.65 postmaster
20135 postgres 20 0 4378m 771m 764m S 2.7 4.9 2:14.57 postmaster
20209 postgres 20 0 4377m 571m 564m S 2.0 3.6 1:14.34 postmaster
20030 postgres 20 0 4376m 890m 883m S 1.7 5.6 3:39.64 postmaster
20171 apache 20 0 370m 30m 16m S 0.7 0.2 0:01.87 httpd
18986 apache 20 0 371m 43m 28m S 0.3 0.3 0:11.47 httpd
19523 apache 20 0 370m 32m 18m S 0.3 0.2 0:07.18 httpd
19892 apache 20 0 380m 37m 19m S 0.3 0.2 0:04.86 httpd
20129 apache 20 0 376m 37m 16m S 0.3 0.2 0:02.39 httpd
20335 root 20 0 15148 1416 996 R 0.3 0.0 0:00.13 top

Thank you
Alex