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Check whether the network is at Full Duplex and at the rated capacity
Its not un-common for Gigabit network cards to be running at 100baseT/Half duplex or 100baseT/Full instead of 1000baseT/Full due to minor auto-negotiation failures.
ethtool is your friend to determine the current network speed possible on a network interface card(NIC).

Kernel Version

Is it greater than 2.6.17. In that case the kernel would auto-tune itself and you don't need to mess around with the various options available.

Here is an excellent explanation of what needs to be tuned for good network performance.