God, gotta love HP, you can't just click on technical specifications, you've go to go through three screens to get to them...Anyway, according to HP
that computer has a 300w power supply. That is not close to the requirement stated by AMD. So Vantharas is quite correct, you do not upgrade the video card without upgrading the power supply. The 7950 draws 200w all by itself, and as the i7 processor in that computer is rated for 95w, there is no way in something we call reality that everything else can run off of 5w.
ASUS Sabertooth 990FX/Gen3 R2, FX-8350 w/ Corsair H60, 8 GiB G.SKILL RipjawsX DDR3-2133, XFX HD 7970 Ghz, 512GB Vertex 4, 256GB Vector, 240GB Agility 3, Creative X-Fi Titanium w/ Creative Gigaworks S750, SeaSonic X750, HP ZR2440w, Win 7 Ultimate x64