Since I don't have any other information to go on, I would say you got PC2-6400 as said in the max bandwidth of the SPD.
If the BIOS thinks it's DR2 533, then you can "OC" it manually. Technically it isn't an OC because it's in the SPD as an acceptable speed.
Check the memory tab and see the speed its actually running at currently. The memory tab will show you the frequency, which should automatically be set to 400, it will also show the timings at that frequency, which should match the SPD timings for that speed. Often the BIOS will set higher numbers (looser timings) than are in the SPD for increased stability, that can be manually tightened.
If you don't know anything about adjusting the timings or RAM speed and it's not set for 400, but is set for 266, then I can help you.
I think that, technically, the RAM needs to default to the speed that they sold it as, up to 800 at least. So if it isn't running at 400 in the memory tab but instead is running as 266 and the BIOS sees it as 533 you could bytch.
Understand, assuming you have a core 2 duo, the processor isn't very sensitive to RAM speeds, so you probably won't notice much of a difference anyway. AMD CPUs are more sensitive to RAM speed.
I would be glad to help you tweak it, post your specs.