the PCxxxx classifications refer to the speed of the RAM, but in a different format. iirc PC3200 is 400MHz front side bus, PC2700 if 333MHz. So:
Answer to first question: The speed
Answer to second question: It would, but it would make no difference. Your Athlon 2400 is most probably a 333MHz chip, and your motherboard is probably 333MHz as well, so upgrading your RAM to 400MHz will do nothing, the new RAM will just run slower that it was designed. However, if your motherboard is 400MHz, and it supports overclocking, you could overclock your processor to 400MHz, and then installing PC3200 RAM would make a difference (not a very big one though)
Quote: "Is there a chart for comparison on which ram would work with what processors"
Well, most parts will tell you what their Front Side Bus (FSB) is, so its just a matter of making them all the same. However, faster RAM is backwards compatible with slower boards and processors, and faster boards or processors are backwards compatable as well, so you could practically throw a computer together, but it wouldn't be working at its optimum ability
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