Well, they're supposed to clear out the leftover crud from apps and programs in your memory, thus "freeing up" your RAM. It sounds neat in theory, but XP already does a pretty good job itself which is why you don't see any performance difference.
yeah i think those programs were desinged for older systems or systems with like 256mb ram... i had the same thing and i didnt see any difference either... the most it freed was about 25mb... seeing as i have 2gigs i dont think i really missed the 15mb that much
i have to agree that the difference would not be that great, infact i used those types of programs that do look like they are freeing up memory, but it was such a resource hog, that i just un-installed it. it was not worth the effort.
i would just rely on getting more ram , and not try to get a program to free it up, you'll end up with double trouble if you have a memory leak. then you'll just end up with freeze up.