Coolant/Overheating Issues

I keep loosing coolant in my 98 Malibu and it's causing it to overheat. It was cooling down by just adding water but now is not even doing that. It has about 79k and I have only had to replace the alternator and brakes. Other than that it has run just fine except for some electronical issues with the signal/hazzard lights. But I don't think they are related. I have heard a couple different reasons why this is happening and I was wondering if any one knew how to tell what is really wrong without spending to much money.

Leak in coolant tank? Intake valve? or the head gasket? Please help!!

4.3l V6? Most likely suspect is the intake manifold gaskeeto!

Plenty of repetitive threads and a couple useful ones on this issue...Some good step by step write-ups on the how to for the 350's too.