Good lens for pet photography?

I'm wanting to buy a new lens for my Nikon d5100, but not sure which one to go with. I mainly do pet photography. So portraits as well as action shots of them playing, running, etc.etc.

The kit lens it came with is ok. But I just think its time for something a bit sharper and so on.

So any recommendations would be great.

Added (1). I was thinking about getting this one: http://www.amazon.com/...004Y1AYAC/

Nikon 80-200 f/2.8
85 f/1.8 prime
Tamron 90
Nikon's 85 f/3.5 Macro

You can go with either of the above mentioned, and 50mm will be a good thing to upgrade but you will be needing something more to it. Just keep certain arenas open.

Hope helped a bit.

Yep, the nifty 50 is a great lens for portraits, but mainly if the subject is sitting still.a 28-80 f/2.8 zoom is also a great choice, since it gives you a wider range and still performs well in low light.

Here is one of the less expensive (but still good quality lenses) in that range:
http://www.adorama.com/...5NKAF.html

Here is a list of a bunch of f/.2.8 standard zooms:
http://www.adorama.com/...8&sf=Price