I agree with the previous post. I have owned several cars with 3800's and sold them with over 200,000 miles on them. I have also known many people with Series II 3800's and have never heard of a 3800 of any year experiencing a head gasket failure.
In regards to the the over heating problems on all 3 series motors...3.1/3.4/3.8, GM now has 3 class action law suits against them for this problem.
GM said "this problem" only goes back to the 2000 model year, consumer reports has it going back to 1995, to which I beleive, I had a 97 bonneville, a friend a 97 grand am, both had to have the intake manifold gaskets replaced.
8/7/06 2002 G. Prix SE 59,000 miles... oil in coolant, intake manifold gasket blown... fyi.
Your gasket problem is caused by the orange antifreeze. This is also what the lawsuit is about. There is a simple fix for this problem, just flush out all of the orange antifreeze and replace it with green. The orange is very corrosive to gaskets. GM has since changed to pink.
Our family owned a 1998 Pontiac Bonneville with the venerable 3800 Series II engine in it. It was a great engine that required little maintenance, but it did blow a gasket once.
I want to buy a Grand Prix, so this doesn't bother me as the build quality of that Bonneville was amazing. I love the 3800 engine - strongest car I've ever owned. Finally sold it due to an electrical malfunction (plus it had too many miles on it for our tastes). But I say that to say this - they are indeed reliable, but the head/intake gaskets have been known to blow.
For the post above: The Bonneville used the orange coolant, so you might be right.