Tuning Effective Cache Question

I have some questions about tuning my effective_cache_size

I have a RHEL 2.1 box running with dual Xeon (2.6 GHz I believe and
they have HT on). The box has 8GB memory. In my postgresql.conf, I
have set the effective_cache_size = 530505 (~4GB).

However, I am noticing on this machine, top is telling me that I have
~3.5GB in the buff and only 3GB in cached.

Here are the exact numbers:
Mem: 7720040K av, 7714364K used, 5676K free, 314816K shrd, 3737540K buff
Swap: 2096440K av, 119448K used, 1976992K free 3188192K cached

1. Is the configuration that linux is running in hurting PostgreSQL in any way?

2. Is there a negative impact of the effective_cache_size being
larger than the actual cached memory?

3. What effect postive or negative does the buff memory have on PostgreSQL.\

4. What exactly is this buff memory used for? We have had a time
trying to find a good explanation of what it means.

Overall, this system appears to be running fine. However, I was taken
back when I saw the current memory configuration. We have been
walking a fine line performance wise, and can quickly become i/o
starved. So I want to make sure that I am not pushing the db towards
the i/o starved side.

Thanks for any insight,

Chris