GMID will get you close to invoice.
Than you have your incnetives to throw on there.
http://www.edmunds.com/ This is where you'll find incentives.
2007's $5500 cash back + $1000 marketing support
or
0% for 60 months/ 2.9% for 72
You can also add in your GM Credit card points as well.
So on an 07 you can save around 11K off stick price on a loaded one.
2008
There is $2000 marketing support + your GMID and GM Credit Card Points.
Make sure you drive both the 07 and 08 though. It's about a 4K difference between the two, but that 08 has a better engine and transmission.
Good luck on the GMID. They are geting harder to come by. Most dealers will get you close to invoice if you tell them you'll wait until you get a GMID. If you get to $500 from invoice on the starting price, than take all the discounts I mentioned, you're doing great!!!!