Well your mainstream American based VAG tuners are the below;
APR -
www.goapr.com - Very well known both in Europe and the States and stage III tuning gives very good figures.
GIAC-
www.giacusa.com - Again another well known tuner in the states.
Neuspeed -
www.neuspeed.com - Well known in the states, but less so than the two above.
Greedspeed -
www.greedspeed.com - I don't know much about these guys but the Yanks do from what it seems.
TAP (Total Audi Performance) -
www.tap1.com - Again I know nothing about these guys but they do across the water
AmD -
www.amdtechnikusa.com - Newbie to the States, but based on the European arm will provide good products.
Forge Motorsport -
www.forgemotorsport.com - No chip tuning of their own but they do offer a number of great tuning products for VAG cars.
These are European brands, but are available in the US at a few places, so shouldn't be ignored;
Oettinger -
www.oettinger.de or
www.roadandsport.co.uk (UK dealer) - One of the longest running tuners for Audi's. A great tuner with a great range of products.
ABT-
www.abtsportsline.co.uk/location.php3 - Again a long running tuner with good products. Official Audi chip tuning for The Netherlands, Germany and Belgium amongst other countries.
MTM-
www.mtm-online.de or
www.qstuning.com (UK dealer) - Well known for good maps with solid performance.
Projectzwo-
www.projektzwo.de or
www.roadandsport.co.uk (UK dealer) - Again a long runing tuner that's well respected.
SKN-
www.skn-tuning.com or contact SKN (UK) Ltd. on
www.scottsmototune.co.uk - Not so well known but good products nonetheless.
Upsolute -
www.upsolute.com - Austrian tuner, well known worldwide
Any serious power mods to the 3.0 lump are expensive, as you'll be looking at a supercharger in most cases, however if it's cosmetic, handling etc. that you're looking at then there are plenty of options to choose from.
Welcome to the boards by the way. /ubbthreads/images/graemlins/beerchug.gif