The faster RAM should just clock itself down to the slower speed. I'm not sure about compatibility, but if there was a problem with this, I'd think the extra RAM would have caused an error during the quick memory test when you turn the laptop on, or would not have been recognized at all.
Also, are those both the actual clock speeds? Some RAM lists an effective clock speed; ie DDR RAM running at 133MHz has an effective speed of 266MHz, since it can transfer 2 bits of data per clock cycle.
There are a couple memory testing programs you can try;
Memtest86MS Windows Memory Diagnostic 
