Not a significant boost, it'll hardly be noticeable. The only reason you should replace perfectly good ddr2-533 ram with 667 or 800 is if your going to be oc'ing. thats where the extra speed will really help!
Since your Pentium D is only running a 800MHz data rate FSB, it only needs DDR2-400 speed RAM running in dual-channel mode to saturate its throughput. DDR2-667 on up are pretty much only useful for OC'd C2D CPUs.