Getback,
if you want a real cheap yet semi-decent card, get a 6570. It has the same specs as the 6670. The only difference is in the core clock, but you can adjust it yourself. Its WAY stronger than the 6450, plz dont buy that card.
However, i would get a 6950, but its at the 250 bux range. It has unlockable components (with a bios update), can be turned into an almost-6970.
Best wishes!