There's low line and high line sat nav rns-d and rns-e (the d has a 5" screen in the middle)
RNS-E was designed for the later B7 model and can be retrofitted to a b6 (I've done a single to double and RNS-E from a seat exeo - the latest RNS-e incarnation)
I wouldn't bother with anything lower than an 'E' and they come in two version mark 1 and 2.
Mk1 have an lcd screen and can take 2 4gb (Non SH) SD Cards. Mk2's have an LED screen and can take 32gb cards.
Mk1 ones are easier to mod (VIM) Mk2's can have full(-ish) ipod integration and AMI.
Neither RNS E will fully integrate with the car - the B6 CCU, etc predates the RNS 'E' so the 'sat nav' element (not the radio or media) won't appear on the DIS until you power down /up or switch from radio / media to NAV on the unit. Simpy the B6 doesn't look for sat nav when it cycles thru the ignition sequence so it doesn't find it. It's a minor issue.
If you have DIS buttons then once the 'sat nav' is recognised you can cycle thru it and the DIS (fuel etc) displays. The radio / track section will appear on the DIS as it does now.
Mark 2 with AMI (the A3 8P type not the A4 B8 integrated, etc) will work / link with mp3 / ipod's and there's a number of threads of people who've done it - it's expensive (c £300 for the parts alone)
RNS 'e' isn't touchscreen and only takes the 1st 5 digits of a uk postcode (you then have to know the street name / number).
Both can have bluetooth and some models SDS - search on here - multi-function steering wheels are also fun / an issue!
I like it lots mind and the look is naturally OEM quality.
link added:
http://www.audi-sport.net/vb/a4-a4-...s/183504-rns-e-2011-mk2-led-my-2-5tdi-qs.html