I'm assuming its for your E90? If that's the case I remapped my last car which was a 2010 320d m sport auto, first of all technically the remap would void your warranty for damage caused by the remap, that said if its done properly.
First of all chipping and tuning boxes are no good, you need an individual remap and there not cheap, I paid £375 but its worth it. As its an auto extra care needs to be taken not to add to much torque, if its done wrong you could be looking at a very nasty bill.
Having said that, if you get a proper job done its worth every penny
, google Simon from emaps, he comes highly recommended on forums