well as far as the overheating goes, try all the simple stuff along the lines of your coolant system like temp sensor, thermostat etc before you conclude it is the headgasket because that is major. The pop pop pop could have been another car, or do you think it is possible it was the sound of an exhaust leak from there being a space in between your header and block? But with the symptoms of smoke and overheating sounds like it could be the headgasket which would suck. If you have access to getting a compression check done than have it done and that will tell you a lot, also, pull out the spark plugs and see if you see any water or oil down there, thats another clue that it could be the headgasket.... If it happens to be the headgasket, and you wouldnt do the service yourself, it may be cheaper just to go with a h22 like you are talking about.....