The answer is simple and complicated. I will try to explain without getting technical.
0db = 0db whether it be Digital or analogue. It is a RELATIVE reading depending on voltages in use.
Peak readings will visually reflect differently due to the fact that digital meters respond quicker than VU meters ( there are special VU meters that respond very quickly in real PRO gear.) If you play a continuous sinewave tone through both and both have the same voltage spec they will both show the same reading. The issue comes between the various levels of gear spec. Your semi-pro gear is referenced to a -10db spec, Pro gear is referenced to a +4db spec and Broadcast is at times referenced to a +8db spec. It is a lot more technical than that, but I think that should suffice for this response.
If you are not mixing bits of gear from each area then you will get a common "0db" throughout all your gear. Each sector has a different "voltage" spec that equals "0db" VU meters tend to show an "average" reading whereas Digital meters tend to show a "PPM" or Peak Program Meter" reading.
Onboard soundcards are usually -10db spec
Up spec cards are usually +4db spec
It is a very confusing issue when you see "0" on your mixer and your software meter is either showing a very low value or sitting at the top of the red.
Here is a software tone generator that might help in setting up your level structure
http://www.nch.com.au/tonegen/index.html
The same can be seen when manufacturers or retailers rate their consumer audio goods
.
100watts RMS
100watts Nominal
100watts PMPO - (Peak Music Power Output)
or just
100watts output
Every one of the above tells a different story about the product.
The RMS reading should be the one you can trust but there are different ways of acheiving RMS ratings, but that is a story all of it's own.
Hope all that blurb helped.
Caveman
digital and analogue audio 
Linear Mode
