the HD5770 is a lower clocked 6790 (6770 is a rebadged 5770 with an updated UVD chip and the 6790 is a higher clocked 6770)
You are pretty wrong dude. The HD 6790 is the 6850 "little brother". Its the same archicture but unly has 16 ROPS instead of 32. Everything else is the same. In performance the 6790 gets far way from the 6770 and its closer to the 6850. The big problem is the power usage, it usses almost the same and in some brands like HIS it uses more power than a 6850.
Higher clocks yes, but also many others features as the 256 memory interface. For kratommonste the 7750 will be better only for the power usage, as it draws way less energy than a 5770 or a 6790 for the same performance.