well the difference between 66 and 33 was negligable
and most say the difference between 66 and 100 will be
the same, but if you have the AWESOME!! new
IBM Ultra 100 75gxp drive, you might as well
get a controller that will squeeze every ounce
of it's spec.
The other advantage of ATA100 is that it leaves
four ide ports (master and slaves) free for various
other devices, such as ide burners, zips, second drive etc.
I think costwise, based on what boards bring on auction sites
when they're still current and desireable, I'd say it might
cost 30 bucks to upgrade in December, which interestingly enuff
is exactly how much it would cost to add a promise ata100 controller
to your board.
but the onboard controller would free up a pci slot for another device.
Course none of this is even applicable if a guy doesn't have an ATA100 drive
yet (read: IBM 75GXP)