For reference, my car is a 2000 5 speed with the T45 transmission. At 60mph, I am around 21-2200 RPM.
70mph I run about 2600, and 80mph is almost dead on 3k. If you have a mid-01+ GT, you'll have a different transmission (if it's a 5 speed), and it'll have a SLIGHTLY better 5th gear in terms of cruising RPM.
If you are torn, there's always 3.90's...