Anything that's memory-intensive will benefit the most from faster memory (usually media applications like video players; I don't know where typical web browsing lies, but since I can browse fairly well on even a PC133 SDRAM system, I'm guessing it's more CPU-bound.) It's never going to hurt to have it, but depending on what you need to do it may be more cost-effective to stick with DDR2, especially if you already have a DDR2 motherboard (they're not interchangeable - I have seen some motherboards with two different kinds of memory slot, but I dunno if yours is such a thing.)
