For a powertap, yes. Other models, no. Start by going to saris', SRM, ergomo, and polar's websites and read everything you can about them. It will give far greater detail than most of us can. Then, bring specific questions here.

Some people use HR monitors with power, others do not. It's probably close to 50-50 right now. Many do not see the need of using a HRM after going power, while others like to compare their power at certain HR's.