If upgrading your graphics card you are correct in that a lot of GPU's will indeed be to big for your PC as you need what is known as a low profile GPU which is much smaller, unfortunately that limits your upgrade potential a little.
Here's an idea of the type of GPU you are looking for. Low profile GPU . Not all of these cards are better tan what you have of course but until you mention what sort of budget you have it is difficult to suggest anything. It might also be useful to measure exactly how much space you have from the slot your present card is in to the outside of your case.