I agree with hard, the 5750 or higher will be a upgrade but any thing less will be a downgrade in spite of higher shader performance. The 8800gts (G80) has major advantages over 46x0 and up till 5750 due to having 320bit bandwidth as well 20 rops.
This is not true at all. The card is a bit weaker than the HD4670 from all the rankings I can find and the benchmarks I've seen back that up. The HD5670 should be a 25-30% increase in performance which certainly is a worthy upgrade with the bonus of DX11 and it uses about half as much power, even less at idle.