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I think that the maximum amount of bandwidth that ide can do is something like 35 mg ... I'm not totally sure, someone want to clarify? >>
Most IDE hard drives won't have sustained reads higher than 35 or 40 megs a sec. (I think my IBM 60GXP gets 39 MB/Sec).
The diff is your burst speed will be higher on ATA 100. I say it would be worth like $20 to me to go ata 100 over 66, but not any more

It makes my system a tiny tiny tiny bit more responsive, and leave me room for faster drives in the future.