I can't remember where I got this, but I like the classification of mildly gifted = 97.5th percentile IQ, moderately gifted = 99th, highly gifted = 99.9th, exceptionally gifted = 99.99th, profoundly gifted = theoretically 99.999th, but practically unmeasurable. Cathy's point is good too, though - when it comes to differentiating levels of giftedness this high, more has to be taken into account than just IQ.