Newbie to DAC's

CD player = Transport + DAC. The computer would be your transport and you just plug your DAC into the line ins. You will need to determine how much you want to spend and what connections are on your laptop, and then people may offer suggestions for you...

to play music from your hard drives you will have to connect your laptop to a DAC you will need a DAC with USB input OR a USB/SPDIF converter to send the data to the DAC

if your Marantz is an older model with a digital output, it will most likely be improved by a newer DAC as well ... just make sure the DAC you choose has enough inputs for your purpose(s)

there are many iPod docks with RCA output to allow you to connect to a receiver/integrated amp/preamp or the Wadia 170i which would connect to a DAC for best fidelity. just make sure your particular iPod is compatible with whatever dock you choose

Jeff

You don't need a DAC to do that...just a cable.
The easy way would be to get a y cable from Radio Shack with L & R RCA connectors on one end and a headphone jack on the other. Plug the RCAs into the back of the receiver and the headphone jack into your ipod or laptop.

Could I simply use the Peachtree Nova? Or do I still need extra hardware?

The Peachtree Nova could be an option ... this would replace the Pioneer