tuning on ec2

I'm running pgsql on an m1.large EC2 instance with 7.5gb available memory.

The free command shows 7gb of free+cached. My understand from the docs is that I should dedicate 1.75gb to
shared_buffers(25%) and set effective_cache_size to 7gb.  

Is this correct? I'm running 64-bit Ubuntu 10.10, e.g.

Linux ... 2.6.35-28-virtual #50-Ubuntu SMP Fri Mar 18 19:16:26 UTC 2011 x86_64 GNU/Linux

    Thanks, Joel

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