memory - Using nginx for WordPress alongside a Rails app

I have successfully configured nginx to serve a WordPress blog (at example.com/blog) alongside a Rails app. My stack is as follows:

Ubuntu 10.04, ngninx, unicorn (1 worker process for the Rails app), spawn-fcgi (for PHP and WordPress), and PostgreSQL.

My question is simple. How much memory should the above require? Currently I have 256mb, and it's completely dedicated to this app. My postgres process has already crashed once, so I'm afraid I need more. Is there a way for me to optimize this? Any thoughts?

I should mention that I'm a developer and completely new to server administration. This is my first attempt at deploying a Rails app to an unmanaged VPS.

EDIT: For what it's worth, here are some current statistics.

$ free -m
             total       used       free     shared    buffers     cached
Mem:           256        168         87          0          0          0

$ ps aux # => just the highlights
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
deployer  2014  0.0  6.4  21676 16880 ?        Sl   Apr29   0:00 unicorn master -D -c /home/deployer/apps/dailyshimmer_v
deployer  2017  0.0 29.6  86112 77856 ?        Sl   Apr29   0:12 unicorn worker[0] -D -c /home/deployer/apps/dailyshimme
postgres  5539  0.0  2.5  43304  6624 ?        S    03:47   0:00 /usr/lib/postgresql/9.1/bin/postgres -D /var/lib/postgr
postgres  5541  0.0  0.8  43304  2116 ?        Ss   03:47   0:05 postgres: writer process                              
postgres  5542  0.0  0.4  43304  1264 ?        Ss   03:47   0:04 postgres: wal writer process                          
postgres  5543  0.0  0.9  44136  2372 ?        Ss   03:47   0:01 postgres: autovacuum launcher process                 
postgres  5544  0.0  0.5  13900  1352 ?        Ss   03:47   0:01 postgres: stats collector process                     

root     12074  0.0  0.4   8724  1096 ?        Ss   Apr29   0:00 nginx: master process /usr/sbin/nginx
www-data 12075  0.0  0.8   9224  2240 ?        S    Apr29   0:00 nginx: worker process
root     30334  0.3  8.3  43600 21812 ?        Ss   00:52   1:08 /usr/bin/php-cgi