300W is enough for your build (If u mind tell me the brand of your PSU). But how much is your HD 6670 GDDR5?
I've found out that HD 7750 GDDR5 priced a little more than the 6670 one at newegg.com. Don't worry because 7750 doesn't need a 6-pin power connector. Its TDP will almost the same as HD 6670, but it would perform much better than 6670.
U could search HD 6670 vs HD 7750 by google, there are many comparisons online. Your CPU might be a little bit weak. Overclock it if possible.