Looking to buy a GXP

[extreme]

I am in the market searching for a Grand Prix GXP. Now I have read a little bit on them and been told a bit on them. I am semi-familiar with the car. My main concern is the year. I was told that there is a big difference between the 2005 and the 2006+? Is this true? What is the main difference?

Also when looking into these cars is there any common problems that should be checked out?

Also while I am at it, the car will possibly get a cai & cat-back exhaust. any suggestion on a cai. also who is the best company for a tune on this vehicle?

Thanks!