your psu has 18A max on its 12V, which powers ur cpu/gpu etc.
total power on 12V = 18 x 12 = 216A
out of which your CPU will use 95W(if not overclocked).. which leaves 121A for the rest of the system,
the 6670 needs a minimum of 66W and max of 92W(if not overclocked). Which leaves upto 30A.. it is really cutting it close however the card will run fine.
ASUS Radeon HD 6670 Power consumption and temperature:http://www.geeks3d.com/20110516/tested-asus-radeon-hd-6670-review/6/
The 5670 uses less power than the 6670 and will easily run on your sytem.. however the 6670 is much greater in performance than a 5670.
get the 6670!