the 800 MHz is faster but you will probably see better results with the 4 GB's of ram. Besides, unless I miss my guess you can't put those in the same machine. 400MHz is ddr while 800mhz is ddr2.
This isn't about RAM overclocking ... If you want to overclock, i suggest you get DDR3 RAM because DDR3 cool down better and is faster than DDR2/DDR1/DDR...