Yes, it should all run at 533MHz. The 800MHz could support speeds of 667 or 533.
Just to make sure you avoid problems, before you insert the 800MHz module, find in the BIOS the memory speed setting. Change it from Auto, to 533MHz, save the settings then turn off the computer and insert the new memory.
* Also after you add new memory, run one pass of MemTest86+ (this should take about an hour or so). If there are errors, turn off the pc, remove your old 533MHz part, then run MemTest86 again. If there are still errors it means the new memory has a defect and you should RMA/claim warranty on it.http://www.memtest.org/
*BTW Memtest would run continously, it doesn't stop, you have to stop it manually.