Unfortunately, this seems to be the case when you find large numbers of sticks. I have a whole bunch of useless old chips myself. For what it's worth, I've found that newer memory is more likely to actually have an manufacturers label on it that says the size... so if you're looking through random RAM at thrift stores, poke through the bags and if there's nothing labelled at all, and none of the boards have many chips on then, then it's probably all garbage.
It did... though I'm really not sure how much of an effect it had. I still need to get more RAM for my Mac, and I read somewhere that it had to be 80ns or better. I've also seen RAM timings listed for Amiga stuff. I've dealt with a lot of random PC motherboards in my days, and I've never had a problem with this though. I have no idea what would happen if your RAM is too slow... maybe just random memory errors or something.
--Zero