The 24MB shared_buffers setting is the conservative default, I'd say it needs to be quite a lot higher for a dedicated database with 16GB of RAM available. But yes, you'll have to restart the server to resize it. http://wiki.postgresql.org/wiki/Performance_Optimization is a good place to start for performance configuration guidelines. Setting the shared_buffers to 4GB or 6GB would seem more reasonable.
Note that on linux you need to adjust the kernel.shmmax sysctl setting (in /etc/sysctl.conf or just by writing /proc/sys/kernel/shmmax) to allocate a block of this much shared memory. If you don't you'll get an error specifying how much was requested, you have to set kernel.shmmax higher than that.
Since you have lots of memory, you might also consider setting the default work_mem higher, which will make things like sorts and hashes (group/order/distinct etc) tend to work in memory rather than using temp files. You don't need to restart the server to do this, just update the config file, reload the service and new sessions will get the new setting. The default work memory for a session is 1MB, you can calculate the maximum that may be used at a single time as
work_mem * max_client_connections and estimate what impact that will have.
You should also increase effective_cache_size to indicate to the planner that the kernel FS layer is likely to be caching a lot of pages in memory outside of postgresql's shared buffers.
etc. etc. hope this gets you off to a good start.