You have to draw the line somewhere, so I reckon that if somebody has taken the trouble to sit down and write a really nice personal condolence letter, that probably merits a hand-written notelet in reply, saying how much their thoughts were appreciated. But I don't think a sympathy card merits quite the same treatment. And if people stop you to say how sorry they were, unless they were close friends and you want to elaborate, I think a gracious "Thank You" is sufficient. Sometimes it's the easiest way of coping, especially if your emotions are running close to the surface at the time.