Relation between RAM / shmmax / shmall / shared_buffers

Hi,
I am having a transactional database heavy on parallel reads and updates. Apart from hardware optimization, I want to ensure that my system parameters are optimized as well. Following is what I am doing on my test system having 4GB RAM. Please let me know if the logic sounds ok (at least at high-level) and if there are other related parameters I should tweak as well:
RAM = 4GB1. Keeping the kernel parameter kernel.shmmax at 75% of RAM (i.e. at 3GB )kernel.shmmax = 32212254722. Keeping the kernel parameter shmall at the same value. Because shmall is measured in number of pages and each page on my linux is 4096 bytes, having kernel.shmall = 786432 (786432 * 4096 = 3221225472, same as shmmax)3. Keeping the shared_buffer parameter in the postgresql.conf file to be 25% of the kernel.shmmax. i.e. shared_buffers = 768MB (25% of 3072 MB)

Thanks,-Bala

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