The bottom line is how much do you want to spend? I've bought 20" Weld Velociti 8's with Kumho 305-50-20's for about $3500. The Kumhos are great tires but get loud the last five to seven thousand miles, plus I only got about 30,000 out of them. I put Michelin Cross Terrains on and get the same mileage, but they come with a 65,000 mile warranty, so I get a 55% discount on the next set.
As far as recalibration of the speedo, you can do that with a Hypertech programmer or get a custom computer tune from Wester's (http://westersgarage.eidnet.org/Index2.htm) or through TByrne Motorsports (http://www.tbyrnemotorsports.com). A custom tune will get you more power, better mileage and if you sell your vehicle Wester's will give you a new tune for your next GM vehicle for just shipping for life.