I want to leave a camcorder recording on a tripod and I want the cam to stay steady and aligned (so that it won't slowly tilt down).

Can it be done with the Manfrotto 128RC or 700RC heads or will I need something like the heavier 501 head?

and it's a Sony PDX10 camcorder: 8 inches long (20 cm) and just 4,5 pounds (2 kg) with accessories.