Check out
http://www.gmcard.com/
If it doesn't have specifics, it will have a phone number to call and ask.

I have the old card--a little over a year ago I had maxed out the 7 year/$500 a year limit at $3500, coupled that with a $2500 rebate and college grad GMAC bonus ($500) and bought a new 2001 Cavalier for $6,500--half off!
At 5% rebate, it's better than the airline mile cards (perhaps on par with the Starwood Card) if you plan on purchasing a new vehicle.

I think the max on a Cavalier now is $1500--not sure what it is on the trucks, perhaps a bit more.