As long as you don't damage your components installing the parts, your fine.
Just make sure you get an HP compatible PSU (they tend to use different connectors). Then upgrade your video card and your done.
However, if you have a scoop over your video card to direct airflow, you'll want to make sure everything fits .
What HP desktop computer uses "different connectors" on their PSU's? The PSU's physical size may be slightly different but the connectors are always the standard PSU ones.