Is it safe to add this graphics card?

My Specs:

APU: AMD A10-6800k w Radeon 8670D integrated! PSU: Thermaltake TR2 430W!
RAM: 2x2GB GSKILL Ripjaws
Hard Drive: Western Digital 500GB
Motherboard: MSI-FM2-A75MA-E35
-CD-ROM added-

I have used many power supply wattage calculators from Newegg, Asus, and another website to determine how much power my current computer is using and they all estimate around 300W of power (rounding up from about 270-280W) Would it be okay to add a Radeon 6670 with my current APU? (my motherboard supports dual graphics) I don't know if a hundred watts of power left is enough to power an expansion card like this, especially when it says I need 400W minimum. Second, is this upgrade worth it at all? I have been playing games like Civ 5, TF2, Smite, LoL, and Minecraft and have decided I need a better gaming experience my APU can't offer. Don't get me wrong, I'm really happy with my APU, it has been doing quite decent on these games, yet I need a bit more power. (A10-6800ks are compatible with this GPU) Also, which one do you prefer?

Possible GPUs link: http://www.newegg.com/...agesize=20

The 400W rating for the card is the TOTAL SYSTEM requirement for typical installs.
You can compare video systems at http://www.videocardbenchmark.net/index.php