My guess is that you'll notice a big improvement. I went from ATA66 to ATA100 and my GXP drives now shine (gained 25% transfer speed, as per atto and sandra benchies). So I imagine that jumping from ATA66 to ATA133 would be even better!.
Just be aware that disk improvements can only be noticed in disk intensive tasks.... don't expect games to run faster (but maybe yes to load faster).
While I agree in part with previous 2 appends, note that it's also true that we're talking about small money here. All in all, my cmd649 UATA100 card costed 35 euro, and it even does RAID0 and RAID1!!!, so for me the performance gains are worth the cost.
Going SATA is a much bigger investment (buy new hw, hdds or cdrws, and maybe a new mobo?), AND performance gains will be not that far from ATA100 or ATA133
I agree with FTC.....for the small amount of cash you get very big returns also if you do alot of burning then it's a good idea to get the burner onto a primary on it's own and not to have the O/s HDD sharring!