i did a 2009 silverado a few months ago. i installed a pioneer double din w/ nav, backup camera, JL front components, a JL audio four channel amp and a 10 inch JL w3 sub. i used the four channel amp's front two channels to power the components, rear two channels to power the sub, and since he shopped very aggressively, he got out for under $2k including parts / labor. the head unit alone was about $800. it was a fantastic setup.
my brother just bought a 2012 sierra that i'm going to tear into next weekend.
my suggestion - don't do two subs. do one powerful one that works well in a standard truck box laying underneath one side of your seat. this will allow you to install the amp underneath the other half of the seat. if you have a sub under each seat, in a silverado, the amp will have to go on the rear wall. this will make for a) a more difficult installation but b) most importantly, it makes the amp hard to service and it gets hot as hell back there.
just my two C
http://www.car-fi.com/news/RetailerYear/Top12.html
check out #37 in the individual section. strangely similar to my sherdog handle...