ring them & just say you made a mistake, you bought pc4200 (533mhz), you need pc5300 (667mhz) or greater
remember... RAM is backwards compatible, 800mhz can run at speeds of 667mhz & 533mhz by default, but 533mhz RAM can NOT run @ 667mhz or greater with no adjustments....
hopefully they will exchange them, you will no doubt have to pay the shipping though..
SK