The number of hard drives and CD writers that you can add really depends on what other components you have in your computer.
Of course, there is a limit on how many components will run off of a power supply. 400W is very standard and only really designed to cope with a couple of hard drives and a couple of opticals drives at best, if that.
You could use a free online tool such as this power supply calculator at newegg to get a rough estimation of how much power your computer would be drawing with everything plugged in. I would then advise that you replace your PSU (power supply unit) with a higher rated wattage. Of course, this tool is one of many and I would look around first before committing to buying anything.
Hard drives do tend to get very hot, to the point that they burn your fingers - this is pretty normal. Apart from the fact they are busy working away which heats them up, it is also due to lack of ventilation within the computer case as the case fans are not always right next to the hard drives to adequately cool them down.