Sorry didn't see your reply to the cpu. With that you could get a 1gb hd 7850 from scan that may outperform the two hd 6670. It costs £119.99. I will just check benchmarks for you. Also whay games do you play?
Also your antec PSU should be fine for a hd 7850 but I'm not sure about a cross fired hd 6670. Are you going to sell the hd 6570 that won't get that much I don't think but it will be something and it could go towards a better gpu.
A GPU doesn't change the quality of a picture. You will still see things the same way if you were using a 10 year old card vs a newer card or even onboard graphics. A new monitor would help you have a better picture though. (assuming its not a problem of having a gpu that doesn't support your monitors resolution or something)