What to upgrade?

upgrade gtx 560 ti to gtx 670 windforce at 400$

overclock both CPU and GPU = beast

agreed, the cpu is fine, the gpu will make a bigger difference, and check the 7970 ghz prices too

Buy a 690ti and do sli.Now Get both speed of 560 and 690 by sli.
Gpu set up look like this. 1x gpu=x16; 2x gpu=x16+x16=(x32); 3x gpu=x16+x8+x8=(x32); 4x gpu=x16+x8+x4+x4=(x32). so 2xgpu=4xgpu.
So insert both gpus in x16slots.

670. Save the rest of the $$ to upgrade cpu and motherboard in summer, after next gen products are released.

That Mb only supports 1x16 and 1x4 PCI-E slots.
@ Tombombadil9: How's the Old Forest doing, and do you still have those nice yellow boots, mate? (Hobbit in joke  ).
You do n't give us your screen resolution, so I'll assume 1080 rez.
Something around he GTX670/HD7970 will do very nicely, along with a improved cooler for the CPU as upgrades-the i5 750 is still a very capable device and howls ou to be overclocked.
As always, check that the powersupply can handle the new hardware, for powerful cards its usually best to use dedicated PCI-E plugs rather tha molex/PCI-E adaptors.
BTW, I'm running a i5 750@ 3.4GHz on a Hyper 212+ paired wth a stock clocked HD7950@ 1080,it can struggle with the very highest setting on the most demanding games-Sigh.