matching proccesor and memory

what is the general decideing factors for you when matching proccesors and memory. i have a p4 640 3.2ghz@800 fsb...... my motherboard will support ddr and ddr 2. but wich speed of memory will be best for my situation..i will watch movies play games and i will do photo editing but mostly web surfing and school work.

 

the speed is dependent on the FSB speed. For DDR and the 800 MHz FSB, it needs to run at 400MHz or be labeled as PC3200. I don't know if it is any different with DDR2. If you get slower RAM, it will clock down your FSB and CPU. If you get faster RAM, it will simply clock the RAM down to the speed of the FSB.