read around on this site - two of the three main stickies at the top of this section are dedicated to exactly that...
What car is this for, what year, etc?
Audi Concert = single DIN
Navigation Plus is most likely refering to the RNS-E.
Assuming this is for an early 2000's A4, you have two main options, RNS-D and RNS-E.... I wouldn't even consider RNS-D (CD based, smaller screen, etc, etc).
Rather than go for the original RNS-E, look for a SEAT Exeo Sat Nav unit... I can't recall the model number, but there are numerous threads on this, detailing exact part numbers, etc (PM me if you're having trouble finding this).
The SEAT Exeo unit is the same as the last of the higher spec RNS-E's that was put into the last B7's (2005-2008)... but it is just as backward compatible as any of the RNS-E units and the dials can be swapped (for about £2 off eBay) for Audi OEM ones, so it looks 100% factory.
I say "any of the RNS-E units" because there are a couple of very minor gremlins with this: the older DIS (small LED screen between the dials) screens that will not allow the full feature set - with my upgrade (Audi A4 2001 V6 TDI quattro, from Concert to a 2006 RNS-E):
1) there was a problem with the telephone book not displaying properly on the DIS
2) I had to power off the RNS-E and switch it back on (3 seconds) to get the directions to display properly on the DIS.
Neither of these was a game changer - it's a still a nice piece of kit that provides some very decent features, although badly overpriced when you consider that you have to buy the bluetooth module separately (£200-£350 depending if you want Audi / aftermarket).
