Find out if the G4 has an "optical" output. It's the same plug as the headphone jack, it serves dual purpose.
If it isn't an intel chipped Mac, you may be out of luck. Then it is only an analog output. In this case, it's a moot point with the 24/192, because it won't output that stream or any digital output.
You could try USB, or converting to spdif/co-axial, but I have no experience with that, and figure the less in the chain the better. And cabling is an expensive bitch of an endeavor. Maybe someone else can pipe in.
If you do have an "optical" out. Then look at a Vandenhul Optocoupler II optical cable ($100). It's well regarded, glass and as far as I can tell probably one of the best you can get for optical. It also comes terminated on one end with a "mini-toslink" (all Macs) plug and toslink on the other. This way no need for an adapter.
If no optical, then you can start with a simple 3mm jack from laptop to RCA "Y" cable into your NAD.
However, I'd recommend this whether you have a digital output or not. Just to experiment.
Go to any store and get yourself an Apple Airport Express (it's a small brick shaped router). There is a DAC in it. It can stream music wirelessly, their "airtunes", from itunes. It can both output an analog signal (use Y cable) or "optical" digital signal.
I actually found this to sound much better than plugging it straight into the laptop with an RCA "Cable" on the NAD. I had the same amp. It sounded better whether I used the analog or digital out (VDH cable).
You may have to contend with dropouts etc (I didn't, just try changing the channels on your primary router), but the sound was decent. The convenience was awesome though, being able to stream music wirelessly. And go ahead using the itunes volume (like a preamp). I didn't find a difference.
It's not the be all and end all, but it is cheap and acceptable sonically. It's not a Michell Orbe w/Koetsu cartridge or anything.
If you don't like it, just return the Airport Express. You could probably find them on Craigslist as well.
The AEx only outputs 14/44.1khz... which most redbook is at. There is a source of contention regarding oversampling DACs. Some think the NOS (non-oversampling is better), staying native at 14/44.1 is better... some don't. Some argue about jitter etc., but just decide what you think sounds good. Let the DIY and manufacturers worry about specs.
One thing you may look at is the Monarchy M24 (I have one). It's a tube DAC and one input linestage (preamp) using 4 6dj8 tubes. I had it using the NAD as the poweramp and the M24 as the preamp. It sounded pretty good.
I don't have the same set-up, use the M24 and Valab as DACs into an integrated tube amp, but looking back I'm not sure if my spending more money really improved things that much. It was my first..ish.. set-up.
You can find the M24 and earlier pre-amps by Monarchy on the used market. Under $800..
Good luck, hope I helped and baby steps..