Personally I don't think there's any noticeable increase in performance between any of the ATA standards (33/66/100) to warrant their extra costs unless it comes bundled w/what ever you're buying at the time. I've noticed the increase in HDs' performance due to faster spindle rate/speed rather than their interfaces but only in those "HD benchmark" programs do I see any differences. I wouldn't spend any money specifically for that alone.
If you're going to get it anyway, I would go w/ the latest and you should be able to get a decent deal on them (ATA 100) from those people who got in on those Staples deal.