I have a 2004 (LS1) GTO. Regardless of which year you get you will love it! Having driven the improved 05-06 models (bigger engine, bigger brakes and different styling), I can say that I wish I had the cash for one a year ago when I got my 04. Many people do not like the stock shifter, it is said to be too notchy, but there are several shifters available. The gearing is the same as the 05 vette z06. I dont know how familiar you are with the GTO's, but some things to remember are:
-The GTO is Australian made by Holden, a GM company. Its right drive aussie version is the Monaro, check out the HSV coupe GTO (jawdrop)
-Dont let anyone say "this one is fully loaded", all GTO's are identically equipped except for trans. and ext./int. colors.
-While being a very stable car with traction control, it's still got 400hp, get to know its limits before playing around too much.
-Some whine from the rear end at ~50mph is normal, a fluid change can usually eliminate or quiet it if excessive.
-While backing, the rear spoiler really screws with your view out the back window, you will develop an intuition of the rear bumpers location, and use the mirrors a lot.
-Don't run any less than 91 octane fuel, you will easily tell the difference, even at part throttle.
-There is a HUGE aftermarket for them, thanks in part to the f-bod's and vettes. Everything from twin turbos to add-on instrumentation is available.
-Most GTO's have tires sized 245/45/17 and are BF Goodrich G-force KDWS, they are a decent tire, but many others are much better in wet and dry cornering grip.
-Check out
www.LS1GTO.com, there is a ton of information there and a user community that is welcoming and insanely knowledgable (kinda like here :thumbsup

Hope that helps some!