upgrading graphics card

You have one pci-e 16 slot for a video card however you only have a 240W pw supply. In addition, you have a sff type case ie it is a small case.

In order to use a real video card, you need to do two things;
1 Measure your case to see if the card you want will fit
2 Upgrade your pw supply so you can actually use a video card

The later is going to be problematic since you have a sff and a standard pw supply will not fit.

In short, while you might be able to upgrade the pw and install a real video card, it is NOT cost effective. It will cost you a lot of money for the pw supply and you will still have an old hp computer.
Here is the spec sheet for that computer;
http://www8.hp.com/h20195/v2/GetDocument.aspx?docname=c04290830