Is replacing my D-70 battery with a Kastar, or other 3rd party battery a good idea. The batteries cost much less. Also, will the Nikon battery charger charge the the Kastar or 3rd party battery?

Be aware of some third party Batteries. There are some really cheap options out there that we have had experience of overheating on. Should it damage your camera your warranty is typically voided. We carry a full line of Promaster Camera Batteries that have a lifetime warranty against defect and have had great results with.

I've never had a problem with 3rd party batteries. I would read customer reviews on the particular battery you're looking to buy, though.