Both of these cards don't need the extra pcie power source so your power supply should be ok. These cards are should be compatible with your mobo but the question is are they powerful enough to give you the quality and resolution you want to play the game? Find out what is the minimum and recommended system requirement for that game then decide which card to get.
If it requires a more powerfull gpu then you may want check if your PSU can provide the power needed by that card.
resolution means your display resolution and/or the resolution you'll be playing the games at e.g. 1280x1024, 1280x800, 1366x768, 1024x768, 1440x900, 1600x900, 1650x1080, 1920x1080 and so on ......(ran out of breath).
the 6670 1 gb gddr5 will be okay for the first 4-5 resolutions i listed. however to play the games you named, check with their system requirements and the gfx cards they recommended. here's tom's gfx card hierarchy chart, in case you need to compare. http://www.tomshardware.com/reviews/fastest-graphics-ca...
also make sure your psu can support the gfx card you choose.