Audi\'s and VW\'s respond VERy well to chips, so that is always a decent upgrade along with a bigger downpipe/catback system.
Chips are anywhere from 399-499 from APR and will yield gains anywhere from 206hp/245lb-ft to 214hp/247lb-ft.
The audi isnt a bad choice at all, just a little bland for my tastes. The 02 or 03\'s look much better then previous years with only minimal change.
As far as reliability, I have heard everything from great to horror stories, so I guess it\'s hit or miss.
The only A4 I have been in was already chipped, full catback, and complete suspension upgrade, so I really cant give you an accurate desc. of accel since the one you check out will be stock.