The P4 is only able to write at a max of 7.5MB/s, so there is no need to purchase the best possible memory card on the market. Any of the memory cards listed here will work.
The only advantage to the faster rated cards is that the read speed is higher which comes into play when you connect the card to a USB card reader that is capable of reading at the higher speeds. This can drastically reduce the amount of time it takes to offload the images and video from the card to the computer.