Hi [NAME],   PHYDR is a Version 4 of mainboard, whereas RGP26 is first version mainboard. It is preferable to replace your R640 mainboard with a Version 4 board. 
Hi, Does anyone know whats diferrence between this motherboards PHYDR and RGP26 for R640 ? Can I replace PHYDR with RGP26? Regards, Salim  
Dell PowerEdge R640