The right SHMMAX and FILE_MAX

Hi. I'm on a 64 Bit CentOS 5 system, quadcore processor, 8GB RAM and
tons of data storage (1 TB SATAII disks).

The current SHMMAX and SHMMIN are (commas added for legibility) --

kernel.shmmax = 68,719,476,736
kernel.shmall = 4,294,967,296

Now, according to my reading in the PG manual and this list, a good
recommended value for SHMMAX is

(shared_buffers * 8192)

My postgresql.conf settings at the moment are:

max_connections = 300
shared_buffers = 300MB
effective_cache_size = 2000MB

By this calculation, shared_b * 8192 will be:

2,457,600,000,000

That's a humongous number. So either the principle for SHMMAX is
amiss, or I am reading this wrongly?

Similarly with "fs.file_max". There are articles like this one:
http://tldp.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/chap6sec72.html

Is this relevant for PostgreSQL performance at all, or should I skip that?

Thanks for any pointers!