It has a very clear advantage, firstly being, it is a newer model, so there will be much less power usage for performance, cooler, etc (the list can go on and on and on). Oh and I think it also has the zero core technology, where it uses 0 (yes zero) power during light use by lower its clock speed and everything. That is one thing I love my HD 7850 for, because I usually snoop around these forums for 4 hours (been on for 3 hours now) if I have the time, and I don't have to worry about my GPU getting worked up
The 6670 was good for its price imo, but it is nowhere near a HD 7750, where you can get it for less than $100. In all the games, the HD 7750 provides at least a 200% advantage over the 6670
Edit: DAMN I WAS LATE, oh well, at least I gave you sufficient help
+1 He told all what you needed to know !