[Build Help] Want to know if I could switch out my current video card with a new one, and if the video card I want to buy is compatible.

Currently using an HP Pavilion p600, it has great everything except for a video card and power supply. What it's running on now is a Radeon ATI 4200 with a 250w power supply. What I want to switch those out with are this and this. What I ask of you if this switch is even possible, if these components are worth buying and how I should go about switching it out. Here is a picture of the ass-end of my computer, I'm assuming these are where what I'd be switching out, but then again I'm technologically retarded so I could be wrong.