Bringing the low-end market up to par.
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On an average the HD 7770 is about 35% faster than the HD 7750, while its lead over the GTX 550Ti is roughly 12%. Meanwhile, if you buy the HD 7770 over the GTX 550Ti, you’re paying a price premium of 23%. The much better performing HD 6850 will cost you anywhere from $140 to $160; making the HD 7770 even less of a compelling purchase.
If you already own a GTX 550Ti, there’s no need to upgrade. If you want to come up from the HD 6770, even then the HD 6850 gives much better value for money. And if you want a low maintenance, low performing card, you can do well with the HD 7750 which also comes in single slot variants.
The only reason I can think of for someone to buy the HD 7770 is that they don’t want to go the Nvidia camp, nor do they want to stick with the previous generation HD 6000 series. In which case the HD 7770 is the only choice you’re left with in the sub-$100 range. AMD simply doesn’t provide enough performance for the price they’re asking. Unless the price drops by roughly 20%, the HD 7770 is a tough sell.