i am sure all links have some effect, i have seen noticable effect from really powerful sites and a slightly less noticable effect from less powerful sites. the less powerful sites would have a smaller effect too.
also google needs to index the links and it may not spider deeply through unimporant sites very often. i think all count some almost immediately, but are weighed in more over time. also there might be a decent delay in Google finding the links on smaller sites.