If your machine is reporting the full complement of RAM, then the RAM has bassed the startup tests. You can run Apple Hardware Test from your OS CD/DVD to do a more stringent test to see if the RAM is OK.
But to answer your unasked question: No, if the RAM were defective, it would simply not show up or it would crash the machine at random times. It would not slow the machine down. Your Mac runs all RAM, no matter what its speed rating, at a fixed speed determined by the memory controller of the Mac. So ALL compatible RAM runs at exactly the same speed.
Thanks
Trevor
CanadaRAM.com