doesn't have to be pc2 4200. Faster ones will work, slower ones will work. It needs to be ddr2 though, and obviously needs to be laptop ram.
But getting pc2 4200 would be the ideal situation, anything slower will obviously be slower and anything faster will automatically be clocked down to 533mhz. (pc2 4200) So if the faster one is more expensive then there is no point in getting it since the computer will not be able to use the extra speed unless you overclock, which I doubt you will do with a laptop.