If the board is a P5B then there's no reason to upgrade, the difference would be so small that you wouldn't notice it.
As fot the RAM, it's some of the best RAM you can get. It's got Micron D9GMH chips, which almost all oft the high-end DDR1066-1200 RAM use, it should de DDR1100 no problems at 2.2V (->stock voltage).