Take the side of the case and have a look to see if there is a spare bay to mount your extra drive
I suspect the old drive and DVD will run off the same IDE ribbon cable, you will need a spare IDE port on the motherboard to run a cable to the new drive and a spare molex power plug from the power supply unit.
to change the cpu you will need to remove the old one and swap the heatsink and fan using new heatsink compound.
Some more information on what motherboard is fitted what exactly is the new cpu is required before we can advise whether the new cpu will work in that board.
SIW will give you all the information of what is fitted in your machine.