I assume you would be replacing the memory already in the laptop since it probably only has two RAM slots. There shouldn't be compatibility problems with higher or lower speed memory. Just know that the memory can only run at the highest speed both it and the motherboard and CPU support.
Also 2GB seems a little high for 400MHz, are you sure it isn't 533MHz or 667MHz?
1) Older laptops can be very fussy with memory. Had two laptops that I had compatability issues with when selecting memory wityh the SAME freq and timing. For my older core2due, I even first emaild the Memory Manuf - how state, Not problem should work great - You quested it, No workee.
2) Most laptops use a "limited/locked" bios and do not allow changing the timings/freq/voltage. This means that most modules that are at a higher freq will still only run at the default speeds/timings.
3) Many older laptops have one memory module soldered in (manuf defualt selling model), and one slot that could be used. In this case it is Highly recommended that the user replacable modul have the same timings/freq/voltage as the soldered in module.
It has been simple to look into all of my laptops but I have a friend whom wasn't so lucky. One SO-DIMM was on the bottom of the machine and another was on the other side, beneath the keyboard and some other things. Neither was soldered in but it was annoying to upgrade it's memory.