Well, two things on this:
1) often times machines from the major brands have a power supply that is just enough for what they stick in. Adding a better graphics card means more power, which sometimes the power supply can't handle. It can be hard to mod a Dell/Lenovo/HP/whatever because of this.
2) This machine is in a smaller case, which means you would LIKELY (im not sure) need a "low profile" card to fit in it. That means your selection is limited for upgrading.
Hope that helps!
It has been a while since I owned a desktop (2002). It has been all laptops since then, but I am thinking of changing back.