Don't believe everything which carries the word "green" in it, it's a hook some companies use to get your money. Anyway look at the HD 6450
for $25. It has a TDP of less than 30 watts and has over 6 times the power of your HD 4200. The HD 6670 has a TDP for 66 watts which considering the stock HP power supply I would not feel comfortable recommending. If SimCity 4 is your most graphically challenging game, you won't notice any difference from the 6450 to the 6670.
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