PC2700 and PC2100 compatible??

What would happen if I put PC2700 DDR RAMs in PC2100 slots? Would they work, albeit at a slower pace? Or they won't work at all (i.e., my system won't recognize them)? The prices are so close now I want to buy 2700.

Also, I have another machine. How do I tell whether I have PC66, PC100, or PC133 RAMs in it? I simply know they are all 168-pin sticks, some are DIMM and some are SIMM. And I also have another stick that simply won't work with these but works on its own. I think it's 133.

If you have some good article/website links, I don't mind reading on my own at all. Thanks for your help!

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