Well, your first problem is that you have a Dell.
If the memory would have damaged your pooter, it wouldn't boot. You'd get a beep code, then nothing would happen. If the computer is POSTing and booting up like normal, the trouble can only lie in the graphical interface. Try using another monitor or another VGA cable, or a different video card.
BTW, don't go to Best Buy unless you know exactly what you're looking for. I've found that they very rarely know what they are talking about.