1.) Should one be lifting to failure, or should you lift at a weight where you can always complete the set amount of reps?
http://www.muscletalk.co.uk/article-cns-overtraining.aspx
http://www.muscletalk.co.uk/article-training-past-failure.aspx
Its good to mix it up between DB and BB, you could switch it up every week, or do BB for a set number of weeks, then DB etc2.) Would it make a huge difference if one used dumb bells instead of barbell for exercises such as bench press, shoulder press, rows, etc.
The smith is not THAT bad, just that it limits your range of motion, and isolates your muscles more, so really for close grip bench, thats not such a bad thing, but I would still do free weight over smith any day, feels a lot more natural3.) I've been using the smith machine for close grip bench, is it really that bad?
Yes!4.) As a matter of course, should one always lift free weights rather than use machines (for example, if the gym is busy)?