ATA133 is just a marketing scheme and is really not any faster than ATA100.
Bingo. Almost. I would go so far as to say that ATA133 is not any faster than ATA66. Current hard drives, even in RAID0 setups, barely saturate the ATA66 bus (and that is in optimal conditions and only in bursts), so ATA133 is completely pointless in terms of real-world speed boosts. And all ATA66/100/133 drives are compatable with each other. The only real time you should
need ATA100/133 over ATA66 is for large hard drives (over 137GB).
I would check the specs in your motherboard manual to see what ATA or UDMA modes it supports, though it is probably safe to say that ATA100 is supported. Maybe a BIOS update will be needed, but more than likely this will just to be sure 48-bit LBA support is enabled (this is what enables the BIOS/computer to recognize drives ocer 137GB). You should be just fine. I would think you need to worry (and only a little) if you are getting a drive larger than 137GB.
\Dan