I'm pretty sure GTX 460 SOC to 900MHz > non-ti 560 > 6850
at those speeds the GTX 460 will cause the screen to flicker randomly and eventually start artifacting.
And if he wanted to over clock THAT bad, he can take the GTX 560 I linked above, which is already overclocked to 860MHz, and overclock it further, it will come close to a GTX 570 and HD 6950 in performance at 1080p or lower.
overclocking a GTX460 from 607MHz to 900MHz will not yield a long life or acceptable stability. a stock GTX 560 overclocked from 810 to 900MHz, or even 1Ghz, will be much less stressful for the GPU.