that's a rather wide range...
for $120~ - radeon 6770,
$140~ - radeon 6850,
$170~ - radeon 6870.
asus, msi, gigabyte, sapphire, his are good brands for amd cards. look for better cooler e.g. dcu ii, twin frozr ii, iceq etc and better warranties and after sales services.
6790 is the most powerful among the ones you listed in the thread header. 7770 and 6790 seems to be on similar level. 7750 is the most power efficient and lastly, 7770 is the most overpriced one. 7770 has received some price cuts, but it still needs to come down a bit.