Both are PC26400 800Mhz. guaranteed to run at the higher speeds advertised by the manufacturer. JEDEC industry standards go up to PC26400. Both will show up as PC26400 800MHz. on your system and will have to be manually set to the advertised transfer rates, 1000-1066 MHz. Basicaly the RAM will run at slightly higher data transfer rates than OC'd PC26400 800Mhz. DIMMs might. Both fall within advertised specs to work on your P45.