A quick snap from my Canon 1D Mark IV taken at 12800 ISO. I applied very slight noise reduction in Lightroom and upon export, reduced the size to 1200 pixels wide.
If I had not mentioned it, would you be able to tell that was taken at such high ISO?
For high ISO shots, it is imperative that you light the shot well. Remember Willems’s Law: “Bright Pixels are Sharp Pixels”.
I shot that with the 1D Mark IV, at 12800 ISO, with the 100mm EF [corrected!] Macro lens, and shot at 1/60th sec handheld at f/2.8. A slow shutter speed like that (lower than one divided by the lens length) needs a steady hand and a bit of luck – oh and shoot ten pictures to get a few very sharp ones. Better still, use a tripod.