Sounds like you need more light, not a new camera. Even with the new cameras you don't want to be shooting cats much above 3200 ISO.

To get more light, you can:

1. Get strobes. Alienbees is my recommendation. Although you might look into cheaper ones like Adorama's Flashpoints.
2. Get more powerful hot lights, like Lowell Tota lights
3. Get a nifty fifty. Canon 50mm, f1.8 EF lens. About $100. Of course you'll have to watch your DOF.