Find yourself a used Panasonic HVX 200 camera. Make sure it comes with P2 cards. This camera shoots in 4-2-2 color resolution, and it does NOT use interframe compression.... both of which will greatly improve on any itty-bitty consumer camera.
Now, if you don't use the proper lighting technique for chroma key, there isn't ANYTHING that will help you pull a good key. Keep that in mind.