GTA IV is both a CPU and GPU intensive game. For a GPU you should be okay with a GTX750 but you need a powerful quad core CPU to run GTA IV at good framerates.
Core 2 quad is not a octa core processor. Currently the only processors used in gaming rigs that are octa core are the AMD 8320/8350.
You don't need an octa-core processor; a quad core is plenty.
Get a core i5 3rd/4th generation.