It prob wont make a huge performance difference going from UDMA66 to 100 because most 7200RPM hard drives have trouble getting over 30M/sec sequencial read/writes anyway. However if you really want ATA100 then you should be able to use the page up page down buttons in BIOS to change it. If this doesnt work then your motherboard prolly doesnt support ATA100, if so download the latest BIOS for it.
Actually Raptors do a bit over 70MB/s, while most 7200RPM drives are a hair over 60. That's from the fastest part of the disk.
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