HD6770 seems to have no direct Nvidia counterpart. Its performance is closest to GTX550 Ti but not close enough to make them competitors.
This is more or less being reflected in their prices as well. HD6770 ranges roughly from US$95 to 110 while GTX550 Ti's range is US$110 to 130 in Newegg.
IMHO, a meaningful upgrade from HD6770 would be a GTX560 Ti at least.