Question : Problem: RAM compatibility.  Would a Bus 667 be the same as a Bus 533

I need a DDR2 553 MHz DDR2 RAM chip (PC2 4200).  The suppliers only has DDR2 667 MHz RAM (PC2 4200) chip.

Are these compatible?

Answer : Problem: RAM compatibility.  Would a Bus 667 be the same as a Bus 533

By definition pc2 4200 ram is DDR2 533 MHz.  PC2 667MHz ram is actually named PC2 5300.
Usually the higher speed ram will clock down to the lower speed , but this depends on the motherboard.
What motherboard do you have?