I just did this job with the help of my brother in law this Saturday. Took about 6 hrs to complete. When I bought my '98 Lumina in 2000, the dealer replaced the intake gasket under warranty due to an oil leak. Now at 115K miles I did it again.
First off, get the Felpro gasket set, and it probably is the more expensive set. There are a few reasons. First, the gasket is a metal gasket with some sort of neoprene around the critical areas. Second, my brother-inlaw is a mechanic who has never had one return to the shop due to leaks using this gasket, and he has used it on his own cars as well.
The Felpro set comes with nearly every gasket and O-ring you need to do the job. We did have to go to the dealer on a quick run to get an O-ring for the press fitted heater tube, but that was only a buck.
We also used an RTV sealant that is used in the shop. Apparently is is much better than the stuff recommended.
Bottom line is there are no leaks, and car is running very very well. The big key here is to make sure all surfaces are clean as clean can be to ensure the gaskets and RTV get good bonding.
I was planning on doing this job by myself, but I am so glad I worked with a professional.