Why would you want Crossfire HD 6770s? (I hope you own at least one of them already, if not both.) Anyway, moderate OC i5 and HD 6770s should be okay on 500W or so.
As explained above, 80 plus ratings are just about efficiency, so if you're going for lower cost, you don't need to pay attention to them. You don't get better quality with higher efficiency—you get designs optimized for efficiency at the expense of cost and sometimes performance. It just happens that consumers expect premium products to be efficient, so the high-end stuff is great on top of being efficient. There are ordinary-quality 80 plus gold power supplies and even 80 plus platinum.
At those prices, get the OCZ ZS 550W (62 euros), a decent-quality mainstream power supply: