I have an alienware with duel channel memory setup. it has 2 sticks of ddr2 sdram, 256mb each at 533mhz. i ordered another 2x256 sticks of memory, also ddr2 sdram, also 533mhz. the only difference is, on my alienware's invoice it says what i have is pc 4200. What I ordered is pc2 4200. I know that what I ordered is the same physical size, so Im thinking it should be compatible, but does anyone know if it might not be? Also ddr2 sdram running on pc 4200 was really rare. (i checked for stuff on newegg, where i bougth my extra ram) it seems that like 99% of ddr2 sdram works of pc2 speeds. I just ordered this yesterday, so I have a few days to cancel it if you guys think it wont work. >.> Thanks a lot.