I agree with Tim, when I first got interested in modding my Buick, I was everywhere on the internet. Mainly random forums (that had no relation to my car), and seeing what other people on CarDomain did to their Buick's (mainly LeSabre's and Park Avenue's).
The forum I found that has been the most useful to me is:
http://www.pontiacbonnevilleclub.com/ — tons of friendly folks who know what they're doing, and lots of good info
http://www.bonnevilleclub.com/ — this is the old site (a lot of nasty stuff behind the scenes that made most of the people move to the above board); I mainly use it for archived information
Since both of my cars are GM H-bodies, and both powered by 3800's (Buick is a Series I – Pontiac is a Series II), the Bonneville Club has come in very handy.
Another good site for GM 3800 powered cars is:
http://www.3800pro.com/