First if you are going for a single GPU configuration then the i5 750 will actually be a good mach for the i7 920 ... see here : http://www.tomshardware.com/reviews/core-i5-lynnfield,2379-8.html
The i5 comes with 16 lanes for a GPU which is exactly what you need and nothing more , whereas the i7 comes prepared for a multi GPU configuration (crossfire or sli) with 32 pci-e lanes.
directx 11 not related to how many CPU cores a game or program uses but to : http://www.tomshardware.com/reviews/radeon-hd-5870,2422-6.html
... the CPU core usage depends totally on the software developer and THAT's why you will find on the market a lot of programs and games that still are not optimized for 3 or for 4 core processors and hyper threading. read here : http://www.tomshardware.com/reviews/multi-core-cpu,2280.html?xtmc=how_many_cores_you_need&xtcr=3
Bottom line : get the i5 750 & and preferable an ati dx 11 GPU offering like the 5750, 5770, 5850, 5870 depending on your budget.