Great comment by Mark Pain (sport professional featured in the D5 review). For sport and action more megapixels means a higher shutter speed and higher ISO. Unless you're printing larger than 20 x 30 or cropping extensively, the very affordable D700 or D3/s can still be great options, especially indoors.
"Even at ISO 2000 the files look sharp with a complete lack of noise or signs of in camera over-processing. Now that the D5 has 20.8 megapixels a shutter speed of at least 1/2000th is needed to freeze most fast moving sport. What many people don’t realise is that the more megapixels you have the faster shutter speeds necessary to freeze the same action. In real camera terms, six years ago I would have been shooting the whole of this year’s FA Cup Final on a D3s on a shutter speed of 1/800th sec. This year I was shooting the match at 1/2000th sec on a D5 - to freeze the very same type of images."
"Even at ISO 2000 the files look sharp with a complete lack of noise or signs of in camera over-processing. Now that the D5 has 20.8 megapixels a shutter speed of at least 1/2000th is needed to freeze most fast moving sport. What many people don’t realise is that the more megapixels you have the faster shutter speeds necessary to freeze the same action. In real camera terms, six years ago I would have been shooting the whole of this year’s FA Cup Final on a D3s on a shutter speed of 1/800th sec. This year I was shooting the match at 1/2000th sec on a D5 - to freeze the very same type of images."