BadLuckAudi said it right on the chipping for the 4.2. Gains are not very big (a few HP). That engine is a rock and can be pushed a bit -- I'd be reluctant to push the 3.0 as much. That said, the primary benefit is probably the elimination of the speed limiter.
Unless you live where the roads are in great shape (no potholes), I'd stick with 17s -- particularly if you're concerned about ride quality. Unless you buy very expensive wheels (strong and light -- two things that don't go together unless you spend money at it), there is no performance gain (larger wheels weigh more) and the ride suffers (many people find 18"s harsh, MOST people find 19"s harsh). Do a search and you'll find MANY discussions on this particular topic.
After you've gotten your wheels, I'd go for either new sway bars or a sport suspension, depending on your budget and how your car is equipped. If you don't have the OEM sport suspension and have some budget to throw around, I'd suggest some Eibach sport springs and the new FSD shocks (see
www.tirerack.com for details). These shocks offer the ride quality of a non-sport suspension but the performance of a sport suspension. Does so by detecting the frequency of the vibration to be dampened (FSD = frequency selective damping) and absorbing high freqency bumps (road surface irregularities) and being stiff for low frequency movements (leaning due to a turn). If that combo is too expensive for you, spring for some H-sport sway bars -- GREAT bang for the buck. Follow the link in my sig to my registry page and I point to a DIY on this if you're so inclined. The combo of both the sway bars and the new suspension would be KILLER. Of course at that point, you'd be wishing you had a 2.7T engine to power it all.
good luck.