Calcium and magnesium sulphates are referred to as permanent hardness, carbonate and bicarb (or the calcimu and magnesium salts of) as temporary. Both contribute to total hardness.
Boiling for 10 mins or so will remove some of temporary hardness (or "soften") the water. Adding Gypsum actually increases the amount of hardness.
Some guy with a broken foot told me that plaster of paris is exactly the same thing as gypsum. He's been doing it for years (though, come to think of it, maybe that's why his bones are so brittle???)
He suggested calcium chloride as a good addition to look into. If I remember correctly it drops pH and brings out sweetness and hop character, without increasing bitterness perception like the gypsum does. He said to beware with calcium carbonate.
why beware of calcium carbonate? as long as one does the calculations it should (and is) fine. i use it in stouts, because i found that i had occasional problems with head retention (which can be a pH related issue if its too low) in these beers. if there is, say a 500g measure of one of either black patent or roast barley in a brew length of 20L then i use 2tsp caco3 and 1/2tsp caHCO3 in the mash, and 1/2 and 1/4 in the sparge water. works for me.
i really ought to get myself some calcium chloride though - ideal for lighter german style beers. i read something once that described this chemical as "the rodney dangerfield of brewing salts". i had to google that to get it.
Beware in pale beers because it raises pH. I might actually look into it for darker beers too Ed. I've brewed some pretty mean dark beers over the last two years and I'm not sure any of them are great head retainers.