If you want, you can look at this Wikipedia entry to start figuring out exactly how much.
So rather than running the memory at 1333Mhz, it will drop down to 1066Mhz and you'll lose some memory bandwidth.
Keep in mind that the system's memory bus will automatically adjust to suit the rated speed of the slowest memory module installed into it. As long as there aren't any other physical or voltage requirement differences between the two types of memory (and there shouldn't be in your case, since both are DDR3 SO-DIMMS), your system should accept it. The older PC3-8500 runs at a speed of 1066Mhz. PC3-10600 RAM runs at a clock speed of 1333Mhz.
