Saw this on Neweggs website, it is GSkills response to a customer complaining about a drop in speed, same as you experienced, after 4gb upgrade.
Thank you for your G.Skill purchase. When maximizing laptop memory capacity, most will need to downclock one speed to operate properly due to a memory controller overload. This is very common. The memory itself will run at its rated specification, but it is unable to since the laptop BIOS is written to automatically downclock if the maximum memory capacity module per slot is being utilized. You should not see much of a performance drop between DDR2-667 and DDR2-800. If you have any further questions, please feel free to let us know.