Uhm, Chilled, GTA IV is probably the most unoptimised buggy game to come to the PC. Hell, using mods which make the graphics 10x better (ICenhancer for example), actually add performance in most areas.
GTA IV is hugely CPU, not really GPU. Its like Skyrim, it doesn't fully use your CPU and that causes your GPU to only run at like 60-80%.
I would go for the 7770 because it is a bit faster, and uses MUCH less power (something like half as much), which in turn means less heat and less noise, and also supports DX11. Its also basically the same price. No question which one you should get.
EDIT: Answering my question in case you didn't understand. GET THE 7770.