Actually, the only thing that matters is the actual performance of the card.
If you think that the core/shader/memory clock makes a difference, take an 8800GTX and a HD2900XTX. The HD2900XTX has significantly higher clocks than the 8800GTX. Does that make the HD2900XTX a better video card? No! In fact, the HD2900XTX has more shaders, a higher bit-rate, DDR4 memory (compared to the 8800GTX's DDR3 memory); and yet the 8800GTX smokes the HD2900XTX in benchmarks. There's just more to video cards than the statistics.
To answer your other question about which GTX460 you should get: it doesn't really matter. The only difference between the brands is the different warranties. I, personally, like eVGA and XFX because they have really good warranties.
I hope this helps!