This has been debated for some time now. There's a post somewhere on here that i remember reading that shows some test results and it seems to be faster with the ATA/66-100.
Back when the ATA 66 just came out, i remember there were some test out there for older hard drives(ATA33) on a PC when using the newer cables, there was improvement. So i'm guessing it would be the same for the xbox.
I changed mine out and i think there's a little improvement, but then again....it might just be in my head.
It's a cheap upgrade, so why not. Even if it increase by .04 secs.