Looking good. A couple things to consider:
1- why 4 gigs of RAM? While I do think you should take advantage of the dual channel ddr chipset of your mobo, if you are planning on running XP, it will slow down your hibernate and resume times. If you are planning on running something like LinuxICE, no problem, load it up. Also, vista/7 may not suffer from the same symptom as XP, but idk for sure, i don't run either of them.
2- You don't have to get the expensive aftermarket harness. Most of the GM-based installs I've seen (mine included) skip it and buy a cheap ebay harness for 10 bucks. I'd only spend that money if somehow the door chimes made me fall in love every time i heard them. They sound nice, but not $100+ nice.
3- Why do you need a powered hub? the ION comes with lots of usb ports. I'd be surprised if you needed another powered hub.
4- What trim cobalt do you drive?
Forgot to mention, I *love* the zotac ION board. Sadly, LinuxICE requires some tweaks to make it run properly. If you do decide to go down that route, I have some chevy theme'd splashes I can share .