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

Nice thing about the Peachtree is the DAC is built in and you get 80wpc if you go with the Nova, 50wpc with the Decco but less inputs. Both are nice and with a Sonos ZP-90 linked by ethernet to your hard drives, you can access all your music and/or play the internet radio stations (extra $36/yr for 192k streaming...well worth it) and you can stuff it into the back of the Nova which is really great.

You also said "yet" when you reffered to your monitors. Have any ideas there? I'm fairly keen to Monitor Audio due to the price/value for what you pay. I'll break it down real quick. Just an example what could be had for not so much.

Peachtree Nova $900 used.
Sonos ZP-90 $350 new ~$275 used
Monitor Audio RX2 ~$850 new ~$550-675 used
Assortment of cables $500 new/used

You'll spend less than $1,500 if you leave out the speakers and some replacement speaker cables. You could easily spend that on a DAC alone. And then you have to worry about your source and additional cables and quite honestly, when you step up to a nicer dac you'll want to upgrade your cables too. Classic snowball effect. Keep it simple.

If you go with an outboard DAC like a Music Hall and a new/used integrated you'll still be around the same price, if not more than you'd pay for the Nova. The Nova isn't the best piece out there. But on a budget? Definitely. I heard this thing streaming 128k interenet music through a Sonos player into a pair of Monitor Audio RX8s. Awesome.

The Nova is very good, but suffers from a relatively poor amp stage. Fortunately, it has a pre-out that enables you to use an alternate power amp. I got hold of a PS Audio A100 Trio here for $550, and it signficantly improved the sound quality. I use a headless Mac Mini via USB into the Nova.