Word of mouth recommendation from a trusted source
Commonsense, (you probably have more knowledge than the average carton seller in a computer chainstore),
Don't overlook the possibility of a computer shop in your locality where if you are lucky you will find real help, and the mark-up makes the total almost the same as a discount net supplier plus courier charges....... They do exist. One in my nearest small town opened <a year ago and knocks spots off the overpriced indifference that existed among his rivals slightly further afield.