OptiPlex GX620 ram speed for upgrade?

I just purchased a refurbed OptiPlex GX620 with Intel Pentium D 3.0 Ghz dual core 945 Express chipset.  It is running XP pro and I have no intention of changing anytime soon.  It has 4 DIMM slots with a 4 GB capacity.  Currently in slots 1 & 2 are Kingston DDR2 SDRAM @ 533-MHz.  In the process of looking to upgade to 4 GB I have come across a little confusion about the chip speed.  The users guide lists 533-MHz and 667-MHz.  A few of the web sites are showing 800-MHz as compatible also.  Is their any benefit to using faster chip speed than the 533-MHz would there be any benefit to putting 800-MHz in DIMM Slots 1 & 2 and moving the 533-MHz to Slots 3 & 4?  Am I asking for problems here?

Thank you for your help & guidence.